hmmm...
maybe juggling everything? simultaneously staying aware of gesture, and form, and composition while drawing. its easy to accidentally make my drawings flat because im focusing on making the page flow - but thats mainly because a lot of these concepts are fairly new to me.
Hmm. Perspective is still giving me a nice challenge. Specifically when it comes to drawing my characters in the environment, and making sure everything is scaled correctly. I'll get it though! 
For me I say storytelling in general. It is making sure I pick a good number of panels, they are clear and interesting to look at, that the shots tell the story well. Most importantly can the reader tell what is happening? I feel like I fight with every new page.
1) multiple objects and people in one panel. Keeping perspective consistent
2) character consistency. Things like hair length, eye color, accessories like belt buckles, earrings, rings, etc. Pay attention to detail!!!
Drawing big. I've always been someone who drew really small, filling up regular sized pieces of paper with doodles upon doodles. It was really hard for me to draw large, especially when I made the shift to digital art. But I knew that I wanted to print my comic therefore it needed to be a print-ready size. So I had to draw the picture on a larger scale. Now this is no problem to me, but it's the reason the first page of my comic is slightly narrower than succeeding pages.
Planning out a story in a way that makes sense and like... writing in general. I think I'm a pretty shitty writer, but I like writing so whatever I guess.
Then again I think I'm a pretty meh artist too. ヘ（。□°）ヘ I guess all I can really take pride in is my update speed. Derp.
Pretty much everything to do with promoting.
You can argue that that is not a skill, but yeah... it is and it takes a a hell of a lot of time I would rather be drawing or writing with =/
Probably the hardest thing for me is pacing. This would be my first attempt at a webcomic with a longer story and I've definitely had to go back and think about whether or not some smaller scenes are necessary.
That, and keeping a consistent schedule. I've been all over the place.
I'd say for me it's the coloring. I guess I could get away with black-and-white cartoons (which definitely have their own unique charm), but one day I just started coloring and coloring, and now I try to color anything that's going to get posted. It's just rather tedious for me, and that's probably the hardest aspect of creating my webcomic.
i can't speak for candel, but for me personally i have a really hard time with pacing
if you let me run loose by myself i'll draw a panel for EVERY change in expression and eVERY nonverbal tic, because i tend to think of comic planning more like storyboarding an animatic when they're two very different mediums
-goji
I agree, colouring is the most challenging. Mind you I am only just starting posting my comic Real Dreams2, so I have a lot to learn
I have yet to post my comic yet, but while working on it I realized I have difficulties with keeping the background from competing with the foreground. It's hard to keep the bg colors and toned done without it looking gloomy and sloppy.
Seconding this. I have the big-picture plot and even scenes of the comic outlined, but paneling them in a way that makes the story flow easily, with pauses and emphasis where important, while keeping the panel layouts varied (so I haven't drawn the same character's face in profile 3 times in the last 2 pages) and making sure speech bubble placement makes sense... that's hard.
(part of the struggle for me is that I watch a lot more animation/movies than I read comics, so I tend to panel things as if they would be appearing in animated format, and sometimes that doesn't translate exactly to a comic.)
Man I getchu on that but I feel the best way to overcome this is just to accept your finished page and done and over with instead of nitpicking on the shapes. Trust me, it's the worst.
I think the main difficulty I face is motivation lmao I want to give up on my comic very often and it's rather difficult to tell myself I'm doing okay enough to draw through it.
Backgrounds are the worst.
Judge me3
I find it fascinating that so many are having problems with panelling and deciding how to actually draw their stuff... For me the problem is opposite. Panelling and drawing is super easy, and a quick fix. Actually planning scripts in a way that makes sense and doesn't make me want to puke on whatever I wrote just so I can't ever read it again, on the other hand, is bloody difficult.
For me it was always writing scripts because I find it incredibly boring and it feels unnatural, it tends to change drastically in the storyboarding and thumbnailing process. Guess I'm just better at telling stories through pictures in general.
Everything tbh. The difference between the (non-remixed) Chapter 1 pages and the Chapter 5 pages for Life of an Aspie are like night and day. Story wise, LoaA is my first time making an original story since like most people here I would assume, I cut my teeth in my youth making fan comics based on existing properties. So far, I feel I've done a bang up job introducing a non standard character like Susan to the webcomic community what with all the praise I've gotten (that I didn't even expect) from people here who can relate to her in some way.
Also, I think the biggest hurdle for new web cartoonists is marketing themselves because with a site like Tapastic or LINE Webtoons, you initially expect to let the site take care of the rest which it technically does so long as you update regularly, but even then, you can only get so far from letting the site do the promoting for you.
I'm in the same boat. I often find ideas I have for LoaA taking on a life of their own so to speak so therefore I never bother with scripts. Plus I find the dialogue frequently changing on the fly anyway.. For example, my latest chapter was much different starting out compared to the final product that my readers saw. The yogini/guidance counselor Yamada and Susan playing The Legend of Zelda: Oracle of Seasons were two things that didn't exist in initial "drafts" for Chapter 5, but their inclusion I feel made the chapter all the better.
Scheduling, storytelling, coloring, and promoting.
I post comics on a Friday, but somehow, it gets delayed almost every week. Storytelling, for the most part, is quite challenging. You have to make readers continue to be interested with your story. Might be because or my laziness. Coloring is kind of time-consuming, and you really need to be consistent with the shadows and perspective. Otherwise, it might look "off."
Promoting is quite challenging as well. Unless you're lucky enough for Tapas to promote your comic. Or when your comic is in a popular genre. (Though, there are still comics that are in a popular genre but still isn't as popular). It is a good thing that Tapas put the Trending section below the Spotlight on the main page, which gives more creators a chance for new readers to view their content.
I guess this could be because I "grew up with" a more demanding sharing culture online that required more active promotion (such as deviantART and various discussion forums I was on), but I didn't really ever expect a site to take care of promotion for me. It will do that once you have built up a larger following and update regularily, but until then you need to do active promotion through social means, sharing and perhaps even project wonderful.
It all depends on what you expect to make of your comic and how much you're willing to invest. The results will reflect that. If you're not planning to do comics professionally, I wouldn't bother honestly because active promotion is only fun if you're a business nerd that enjoys the action of promotion and studying how various techniques and actions affect the flow of the numbers. If that's not your thing, I suggest you just sit back and do your comic because you like to (: How many people you've got reading it or not doesn't matter if the main reason you draw is because you love it anyway.
Upload on multiple sites! Webtoons or smackjeeves are the most common alternatives besides Tapas, but aside from that you could also try tumblr and deviantART! Different comics find different levels on success through different means (: I know in my case, for example, smackjeeves isn't very effective at all. Tapas, however, has always been effective for me. Even on my less popular types of series (for example I have a horror series). Other people experience the very opposite.
