I love the painting! Makes me wish I could paint traditionally like that!
I think that it looks flat is because the perspective doesn't make sense. In order for you to see the entire grass field is if its a bird eye's view. I also see blue reflected light on the tree, the tree house and on the woman's shirt. But where does the blue come from when everything is surrounded by green? Where's the horizon?
So here I added a horizon and a blue sky so that the blue reflected light from the tree, tree house and on the woman's shirt makes a little more sense!