My pixel brushes have stopped building on top of each other. No matter how many times I go over it, it won't get darker. Any way I can help this feature?
Hopefully I've understood what you mean.
Pixel brushes look lighter because not all pixels are filled on the initial stroke. As you continue to brush over the same area more pixels are filled in, giving the impression that the stroke is darker. Once all pixels in that area are filled in and there are no pixels left uncoloured it will not get any darker.
If you zoom into your stroke is it almost completely filled in or are there a lot of white (or what ever base colour) underneath?