Honestly I think what's making it look a bit flat looking is that shadow overlay you have over the second panel. When doing a flat colored MSPA style, the drawings are inherently "flat", but they don't necessarily feel flat. When you add a straight gradient shadow like that over it, however, it draws attention to how simple the coloring/shapes in the style are and MAKES it feel flat. I'd either change the shadows to being cell shaded or just do away with the shadow entirely.