Ran into a problem where I had two layers, added a mask, and discovered very quickly that with the mask layer selected, I was unable to use the certain Brush tools to paint on the mask, while I could others.

Turns out the solution is pretty simple. Turn off the Protect Alpha checkbox in the tool bar.

It's not obvious to me at the moment what this would even accomplish on a mask layer that is represented as a gradient from pure black (transparent) to pure white (opaque), but the brush setting certainly matters.