Dark squares and trouble painting landscape

Currently I’m trying landscape painting for the first time and I’ve got some problems.
First of all: Since I created the landscape there’s been dark/shadowed squares on it that wont disappear.
Secondly I can’t seem to be able to paint everywhere on the landscape and it’s very unpredictable. I have areas which I can’t change, sometimes I can’t paint anywhere and the paint is often applied in big squares.
I’ve got the layers in my landscape: rock, dirt and grass. At the top of the picture there is a gray/green area on which I can’t paint and it’s not even the colour of any layer in my material.
The grass is also untouchable and I was able to paint the rock texture you see att the bottom, but now I can’t touch that one either…

How can I solve this?