My Adjustment brush has stopped working

I use ACR quite a bit, but now the Adjustment Brush has stopped working. I can set the starting point but dragging the brush over an area shows no adjustment happening—regardless of the adjustment option. When I check all my adjustment points, none show they cover any area, all I see is the entry point marker. The gradent adjustment still works. I read through some other posts with this problem and an update seemed to be the solution. I updated my ACR so now are running version 12.4 for Mac. Unfortunately my adjustment brush still does not work. Anybody know of a solution?