Hello! So I'm definitely not as qualified as the more successful artists on webtoons, but I've been on there for almost a year and I had an okay following of about 1000 subs a month ago and then I'm pretty sure my story got featured in one of the collections on the app and it jumped to 2800-ish subs. I also struggled with engaging people to read my story, unlike Tapas webtoons doesn't have profiles for authors so it's harder to link back to their work if you just read a funny comment and want to go look at their profile. So some things I did right before I got featured were:
make your thumbnail art a m a z i n g, still close to the original style, but still on the better scale of your drawing abilities
post often (every week is hard but it does make you more visible), I hear posting in the evenings helps a lot even though I post in the mornings (I think it all depends on what your time zone is)
engage in the comments section, my readers have said they like when authors do this
if you can, host tiny contests, like fanart giveaways (I gave out pinups of one of the male leads for guessing the number of butterflies drawn on a page)
keep positive in your comments and author notes, sometimes its hard but the readers you have will appreciate it!
experiment with your art and the scrolling format if you can, a lot of my readers have said they like this and it better engages your audience on the platform you're on
I hope some of these help, and I totally understand the frustration, but if you're not losing anything from being on webtoons, I recommend still going with it, link people back in the forums and be active in reading other people's works, not only does it support fellow artists, but looking at different art style and layouts can help give you idea for future pages 
EDIT: Also yes, what @demikuro said, do fanart of other peoples works, or trades or even contests! this is all very helpful to engaging people (although potentially more effective on Tapas since you can visit profiles on there)
