I think it's time to move on and grow the OC's age up. Certainly is your style making them cute and lovely !
The grown ups I've seen, seem like children with just a long body and mostly I've noticed you hide body features so much with a cloth.
I don't think you shall start drawing boobs lol, but the human body isn't always plain (of course there are chances with flat chests; hey! both male and female!). No one would judge your draws as indecent if they show some features either, we were drawn like that xD What? You would want humans with just clothes on?
Then the last occuring thought is that few times you notice your draw smile in an unique way, I often notice a similarity. ( ex. ) nice faces no?
The eyes are now important too, with the brows too. Draw some very sad faces and I will be happy to see them xDD
Lol but I've been mean, you've just started drawing, don't push yourself so hard, but never give up to give your best :]

I think you just have to work on drawing more realistic people, it might not work with your drawing style to draw older people or more realistic people, but drawing from real things really does help in the long run.
try drawing from photos maybe? thats what I do. I usually just look around dev. art for photos of objects, people, buildings, whatever I wanna improve, find a good photo and try my best to draw it! ^^

oh and posemaniacs.com is a great place to work on anatomy and stuff x3
keep up the good work! :3

I prefer the old saying "practice makes perfect," although reading up and studying wouldn't hurt.

(Sorry I haven't commented on anything you posted yet... ) But I don't you need to improve at all! you have great artwork! :] Probably just drawing backgrounds and anatomy, you know, the stuff every artist has problems with. 8D