Sorry if this a really silly question, but here goes. It seems as though I can mask on the Back and side planes just fine. When I try ad mask on the Bottom plane, nothing happens. It doesn’t matter if I am “Adding” or “Subtracting” a mask.
I am on a Mac if that makes a difference. I have searched these forums as well as Google trying to find anything that resembles this issue.
Any ideas? I am really new to ZBrush so this may totally be a dumb question 
Hide the other planes by ctrl + shift clicking the one you want to work on, for instance, the bottom plane. (mac equivalent obviously, not sure if it’s the same, command click or whatever)
Then you can clear whatever mask may be on there from a fresh shadowbox, and begin drawing masks directly on that plane alone. When you’re done, ctrl + shift click outside the shadowbox on the canvas and the other planes will reappear.
This is a good practice to get into whenever you’re using shadowbox in general as you avoid weird masking problems and “spillover” from masking on one plane to the next simply by ctrl + shift clicking the plane you want to mask first, and each time thereafter.