Fenris nailed it! The way to get a following on Tapas is to go in with the knowledge that you are in the ring with professional comic creators and to realise that means you're going to have to try your best to look as good compared to them as you reasonably can with your available time and budget. It's all about hard work and attention to detail. A good marketing campaign will only be effective if the promo artwork and logo used look good (so people click it) and then it links to a comic that looks good and is easy to quickly get invested in (so they stay and read it and subscribe).
I have currently just shy of 1400 subscribers on Tapas. I hit 100 after about three months of posting without engaging in sub-for-sub. Do you know what most of the people I know who have over 1000 subs have in common?
Professional training or experience.
This is seriously something I wish more people were aware of! Nearly all of them either have a degree in something like illustration or animation, AND/OR have done some years of working as a professional illustrator or similar. Almost none of them it's their first ever comic, most of them have at least made some short comics before or had an older webcomic online.
So when I came to make a comic on Tapas, I knew I was going to have to really work my arse off because I know some of the people with popular comics on here, they're IRL friends and aquaintances and they're incredibly accomplished and skilled. Like the artist on BREAKS is literally one of the most famous comic artists in the UK of her generation, she's done work for Marvel, she drew the official Life is Strange and Jem and the Holograms comics, the graphic novel adaptation of Vampire Academy and she's regularly a special guest at comic events... or the creator of Firelight Isle is a Rising Stars of Manga first place winner and was the artist on Freakangels, like yeah, as in, that anime that just launched on Crunchyroll? He drew the original comic that was based on (thankfully unlike Warren Ellis who wrote FA, he's a really lovely person).
I've worked professionally as an illustrator and games artist, had my work published a few times and placed in some national manga competitions, but I'm seriously small-fry compared to people like that, so I've been working hard to create as polished and entertaining a comic as possible with a reliable update schedule, and also to make my marketing really consistent and polished too. It's not easy!
People on the forum might give your comic a look just because you seem like a nice person, but the readers on the app don't know anything about you and are only interested in how good it looks and how easy it is to get into. The upside is; they don't know what school you went to, whether you've won awards, been published, or how many readers your last comic had; you are on an entirely equal footing with pros and it's all about your cover versus theirs! The downside is those pros have years of experience to draw on and they already know exactly how to get and hold attention with their comics. This can be daunting, but don't forget that with the internet, there are amazing resources for improving art, design and storytelling, and with practice and hard work, anyone who puts in the time can equal a professional-level comics creator! 
Your covers should always catch attention with things like colour or faces, and they should always feature the title written in a clearly legible way with some kind of nice and appropriate typography. They should ideally in some way make it clear what the comic is about. Lavish covers with attention and make sure they're polished looking.
The first three updates of your comic ARE VITAL. You need to establish interesting characters and the art style and launch into an interesting scenario immediately (this is definitely something I'd do differently if I could start again on my comic, I'd have the opening be faster paced). Three episodes is roughly how much a lot of readers will give a comic a try for, and if you haven't hooked them with the combination of that attractive cover and a strong, interesting start, you've lost them.
Also the comic should be very easy to read on a small screen, so always pay attention to your font size and spacing and your speech bubbles as well as the font choice. Anyone who doesn't know about comic fonts or where to start... just pick a simple free dialogue font from Blambot and you pretty much can't go wrong. Anime Ace 3, Ditigal Strip 2 or Letter-o-Matic are my suggestions for people who are stuck.
Assume you are trying to attract a person who has so many good looking comics to choose from that they will give up at the first even minor inconvenience. "This cover doesn't suggest the comic is about anything interesting, I'm not clicking it", "This art looks scrappy, I'd rather read a prettier comic. Bye!" "I can't read this writing on my phone, I'm out", "I've been reading three whole pages and nothing has happened, see ya later, space cowboy!" (feel free to read all these in a Teen Girl Squad voice if it makes you feel better. It always makes me feel better! 
).
Some creators might just say "Ugh, well, I don't want people like that reading my comic anyway. I'd rather have smart readers who look beyond the surface of my comic and see its depth and meaning an also have an attention span for slower-paced, atmospheric storytelling!" which is fine... but not necessarily compatible with building a big audience on a platform where the average reader is a 20 year old woman who is a casual comics fan (ie. no, she probably has not read Akira, Watchmen, Persepolis or Maus), likes Romance and/or BL and is reading comics on her phone while eating lunch between uni classes or something.
Tapas isn't an art community like Deviantart, where most interactions are artists wanting to connect with other artists by appreciating each other's work. It's a publishing platform where the vast majority of the userbase (like literally over 95%) are just here to read comics on a mobile app. Chances are you can reach 100 subs by just slowly collecting people on the forum and discord who want to be friends, but if you want to be able to get beyond that (and you probably will because guess what lies behind that progress bar counting up to 100 on your dashboard? Another freaking progress bar going up to 250), you'll need to make a comic that appeals to the non-creator readers who came to read pro-quality comics.
also my fault for neglecting my series for few years.
