Fancy colors. Like the person below said it does seem flat, there's a theme that runs through all your art which is that every character you draw looks like a standalone figure (kinda like an amiibo) and that becomes very apparent when you put a bunch of character together like this. There doesn't seem to be a focused light source, and they're all standing at the same 3/4 angle which takes the life out of them a bit.
Challenge yourself to do more unorthodox perspectives. Draw a character from the back occasionally, or the side, top or bottom. I will come in handy down the road.