Maybe the plants are too blob-ish.
Have you ever looked at nature from afar? I don't mean on a postcard or screensaver either. I can only think to describe it as crisp but out of focus.
It seems like you just layered colors instead of actually creating trees that are out of focus, if that makes any sense.
For example in the right hill there's one tree at the top just before the space followed by more trees. That one tree looks like more tree than all the others in the middle of that tree-line.

Then again I'm not an artist so I'm not really helpful.

I say U + me Collab RN