Sometimes I can't come up with a witty comment or I feel that I don't have the time if the comment I have in my head is long.
Part of it is because I've been burned one too many times over the years. I'm not good at wording, so a mini-essay comment that I genuinely meant as a compliment gets mistaken as a criticism or something else entirely. I'm sick and tired of that happening. I'd rather not comment than have that happen again.
Oh gosh, usually for me it's I can't remember how to spell the dang name (it'll be a crazy fantasy/Sci-fi name too), so I feel like if I spell it wrong the author will murderize me. I see it's a pet peeve of a lot of creators on the forum for people to get a character's name wrong, and I don't want to step on any toes. ;u;
I usually don't comment because there is no reason.
Usually my comments are like a reply to the authors description, so if there's nothing to reply I often don't comment, cause it feels like i'm just reading instead of interacting with someone, also If the story isn't catching my attention or got me lost I won't comment, maybe just something like; "god i'm lost, gotta re-read the whole thing" So basically if there isn't any specially interesting in the page I won't comment, I need to see a cameo, there should be a plot twist, an improvment or an interesting composition overall, I need to note something in page to comment, otherwise it's just meh, and actually over the time I'll stop following the comic.
It indeeed bothers if the author does not reply (in the case of comics with little fan base) but for me isn't kind of a big deal, it bothers but i can rol with that, An example of a great author that is commited to feedback is Harkill, in his comic HeLL(P)3 he often replies to coments, his description seems also like a coment or a gag about the comic itself, and there's always an easster egg on every page, everything going by the hand with stonnishing and marvelous visuals, it just clicks
I mostly don't comment because I can't think of anything to say. I'm not one for speculation or reading deeply into the story because I'd rather let the story unfold and give the author a chance to surprise me. (Besides, I don't want to be proven wrong anyway 
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ppl have got the names wrong for characters in my comic in comments before and it is a bit of a 'oh no. i havent been clear enough. how do i fix this' moment, combined with not knowing how to be like 'oh actually its not x its y' but id rather get them comments than no comments.
now getting pronouns wrong. thats happened too. and i had No Idea how to correct it so jus ignored the comment instead.
I only comment on significant pages with drama or something interesting happening. One comic page is only a fraction of the whole thing so not every page really needs a comment. It's common to see more action/suspenseful/interesting pages to get more comments than the slower paced pages.
If I don't comment, then it's simply because I don't feel like it.
But there's sometimes those moments I think I should say something, but I can't find the right words at the moment. So I leave it be, but come back a few hours later, when I have a better idea of what to say.
I comment when I feel something strongly! So like, if a page is incredibly sweet and I'm touched, I'll try to remember to actually say "omg this is so sweet!!" If two characters are bickering, and their back and forth is a lot of fun, I'll try to remember to comment "oh my goodness I love this." I know as a creator, getting a ton of "HOLY CRAP WHAT" comments when something big happens feels amazing, even if those comments are small and don't leave much to reply to, so my goal in commenting is mostly so that the creator can know that I felt a thing.
So if I don't Get A Feeling, I won't usually comment. Also sometimes I just forget!
Like joanne, I don't really have fun speculating because I'd rather wait and see what's going to happen, and writing little mini-essays on the composition would feel forced from me because I don't really experience media that way -- I look for stuff like that when someone asks for a critique. My reaction isn't going to be "whoa, the colour contrast just perfectly hilights the points of interest and sets the anxious mood of the scene really well" -- my reaction is gonna be "DANG THIS IS PRETTY AS HECK, I love these colours!!"
though I am the kind of person who will page back through the comic to remember a character's name xD
Me too! I got a name of a character wrong once and the author thought I gave the character a cute nickname... That made me feel so bad for some reason. 
So ever since then I scroll through older pages to make sure I got the names right 
I was never good at commenting on stuff, even if I really appreciate a comic. It's a mix of being lazy because I'm overworked and I only read comics whenever I have a break from said work. Other than that it's because I'm not sure what to say because I more often than not feel like there should be more weight ehind my words. Personally I am overjoyed when I receive even the simplest of compliments, so maybe I don't need to think so much about it all the time? What do you guys think?
I usually comment on details in the page that I liked, artist's comments, or when I feel like I have to say something about the progression of the story or character development, or simply when I want to complement the artist.
One of the most common reasons for me to not comment is when I feel that it'll be flooded in the other comments, so the artist isn't going to read it anyway (so why bother ). The other most common reason is that I don't actually have anything to say. I don't want to force myself to say something when I actually don't. ^^'
Same, that's my main reason.
But sometimes I'll comment when the page's art looks beast. Obviously, I won't do that every update. But every once in a while a creator will make a page that just looks so sick. Like, you can tell they put more work into it. It's a good way to leave a meaningful comment without remembering names haha 
I truly never thought about the names issue. I write and compose the comic as it was meant to print (because I do print it), so I make sure I name them often enough throught one episode, but still. Reading online is weird.
I often feel useless when commenting. I don't have anything particular, or witty, or interesting to say, so I just like my stuff and that's it. I felt the same towards forums. Now that I'm a creator and I'm trying to build an audience or at least some light relationship with readers and other creators I'm learning to comment more often - even if the comment is still very short ._. I guess I'm too introverted, but I can improve
Like a lot of you have said, my main reason for commenting is not knowing what to say. I get shy and worry that I'll say something weird, but I try to force myself to think of something to say if I genuinely like the comic, because I want the artist to know that and I'd want someone else to do the same for my comic. Even if a comic already has a ton of comments, it can't hurt to leave one more to let the creator know that another person enjoys their work!
As a creator though, I can say that comments generally aren't annoying, and you shouldn't worry about saying something someone else has already said. If two people say they like something about a page, that makes me twice as happy! Same if the same commenter says they like multiple pages. As a creator, it's so easy to forget that there are people reading your comic who don't leave comments, so even just a "nice!" or even an emoji can be a nice reminder that someone is enjoying your work.
For me a like is more than enough, also I am lazy. In my case I like it when people just like a page because I feel that they took the time to check out my comic 
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I only comment when is something that really caught my attention.
