When I paint on a layer mask I paint with Black and White paint with the brush set to a low opacity and blending mode overlay. Only gray area can be painted on when blending mode of the brush tool is overlay Black and White area will not change. When are area go from gray to white or black you can not longer paint in the area that changed from gray. I do not seem to have any problem painting gray area to Black or White. Paintbrush paint build up it may take many strokes for gray to turn Black or white.
When I use the Clone stamp tool I normally clone into an empty layer at 100% opacity normal blend mode align sample this layer and below. I can always adjust the clone layer with Photoshop's tools and features. masking, blending nudging etc. When You use the clone stamp tool be sure you check the tool option bar is opacity is set to 100% and blend mode st to normal and check the other option as well..