I have been having almost the same problem. My adjustment brush doesn't seem to be working and the mask overlay isn't showing at all even if I hover over it, check the box or hit O. I can't seem to find an answer anywhere and it is really impeding me work. Were you able to figure anything out?
Select the Adjustment Brush and paint. Hit [O] to turn on the overlay. Then hit [Shift][O] to cycle through the colors of the overlay (Red, Green, White, Black) Do you see the colors cycle? If not, check your Flow and Density settings on your brush and make sure you aren't set too low so that you are painting too faintly to see.
OMG I was about to say I've already done all that but I double check to be sure and the flow was at 1!!! You're brilliant! Thank you! That must have somehow gotten bumped down by accident!
thanks! It was driving me crazy!