This is probably not going to be a post you're going to be upbeat and cheerful about. I'm sorry.
1.Sometimes all the hard work in the world doesn't help. I know that's a bitter pill to swallow, but it's true. Sometimes something you've put a lot of effort and love into just doesn't connect with people, and that's not your fault. And it's nobody else's fault either. It sucks, but sometimes that's the way it goes.
2.Putting anything original on the internet and trying to garner attention for it is hard. Fanart of famous IPs, for example, will ALWAYS get more attention than your original stuff. I've got a scribbly, unfinished piece of fanart I made for Beauty and the Beast on Tumblr - which I dashed out in 40 minutes on a lunch break - with over 8000 notes on it, whereas something original that I've poured my heart and soul and love into for hours is lucky if it breaks above 10 notes. It's just how it goes.
3.Not every comic will work on every platform, and you don't know until you try. It might be true that longform comics do better on LINE than on Tapastic in general, but your comic might be an exception to the rule. It happens! That doesn't mean your comic is bad, or you're doing something wrong.
4.Maybe your comic simply occupies a genre or uses a certain set of storytelling tools that means its target demographic is naturally narrower. There's nothing wrong with that - it just means finding an audience is going to be harder than if you did something more crowd-pleasing.
...I think I might be one of those examples you speak of. I've never been a Staff Pick, I only got a front-page feature AFTER I gained 1000s of subscribers, I've never had a Daily Snack, etc. I got where I am mostly on the back of my own effort.
... and I want to tell you that just because you can't SEE the blood, sweat and tears doesn't mean it isn't there. I put a LOT of effort into being as visible as possible, tweeting and posting on Tumblr about my updates, drawing guest-pages for my friends' comics, I go to cons where I meet people in person and talk them into reading my comic, I participate in every comics-related event I can possibly manage - weekly hashtag-events on twitter, holiday events on Tapastic, submit my comics to anthologies, etc., etc. And that's just the marketing part; that's not counting the endless hours I put into drawing it and posting it regularly. It's just that a lot of that effort might not be visible from the outside.
And just because I have a bunch of readers NOW doesn't mean I wasn't exactly where you are once - it just happened a while ago. I spent a DECADE posting art on the internet to mostly crickets chirping and no response. It was immensely discouraging, but I kept going anyway. You really do have to keep rolling that boulder up the hill in hopes that one day it won't roll back down. I know - it's how I did it. Quite a lot of the people you see launching a comic and sky-rocketing to high readership numbers probably already had an audience elsewhere.
Well, firstly, here's the thing about doing nice things for others in general: you do it because it's a nice thing to do. Nobody actually owes you anything for it. Even less so when it's a virtual stranger on the internet. If it's a very POPULAR stranger on the internet, it's very likely that they are very often subjected to people doing nice things for them in the hopes of promotion, and it makes them even LESS likely to return the favour - they've learned through experience that you might not be genuine in your effort.
People will RT or link back to things they genuinely like and enjoy, not out of a wish to do you a favour. It sucks to hear, but that's how it is. And if you're trying to establish connections to people who have those millions of followers you speak of, well, it's entirely possible that your tweet either got lost in their mentions, or maybe they simply didn't have the time to read your comic and thus don't want to RT anything they haven't seen for themselves.
And, finally. This is the bit no comic artist likes to think about, but it IS necessary to consider: If you think there's something about your comic that isn't working, take a good, hard look at it and try to make changes. Do you need to step up your game somehow? Does the artwork need polish? Is the storytelling good enough? Is there anything you can improve at all? Is there ANYTHING about your comic that, at first glance, might turn potential first-time readers away from it? I haven't read your comic, so I'm not telling you there's anything wrong with it - I genuinely don't know. But it's something we all need to think about when we put our comics out there.