(I already follow you so I guess I prob should have just checked first, thank you for linking anyways tho!)
I took a look at the twitter - I don't think there's a problem with your art, and part of the 'problem' is the amount of retweets you've got going on. The other possible issue is the #'s you're using, #artistsontwitter and #originalcharacter(s) are both really busy #'s.
A piece of advice I've seen and used a little bit is to use a mix of popular and niche #'s on the same post. Your stuff will only be on the popular #'s for a little bit unless it gets a lot of engagement, meanwhile, the smaller #'s you'll be there longer, but with less eyeballs. The theory is that the engagement you get in the niche # will boost you up in the popular # and vice versa.
Social media stuff, like webcomic stuff, is a long con, and a lot of it is trying out different techniques and strategies to find what works for you.
The key thing I try and keep in mind is what I said earlier, your audience will grow based on what you post regularly, and they will engage with what you regularly post.