Hi Grampy, that's a nice work and the answer is a simple one: "Value" is a word that the art community use to defeine density of light and shade in painting. Don't be afraid to highlight some small areas in white or very light paint for brightness, on the water, tops of the rocks, edges of trees etc, and add a touch of darker colour to contrast it.
As I said, it's a nice work and needs only a few small touches to finish it. Hope this helps.
Jim.