Hey! Beautiful illustration! :) I think the simple answer to why the illustration ends up looking flat is that you're only adding flat colors without indicating the form or volume, either through linework or lighting. In this case, I pushed the illustration further by adding lighting to give the character more volume. :) You can also achieve volume by painting with midtones alone and adding color variation, without needing to paint more dramatic lighting. The key is to emphasize volume, whether through linework or painting. Avoid thinking only in terms of flat colors and flat shapes. Hope this helps!