Visual Display not visible on Mask

First of all Thank you for your help.

I have a random issue with CS5.  Every now and then when painting out on a mask transparency, the thumbnail visual does not show a representation of what was painted black.  It is as if there is a delay with CS5 registering the information, but my Mac system is new and bawny. The weird thing also is that I happens on my computer in the studio as well as at home. 

The only change I made to my preferences is changing the color of my transparencies to a solid white. 

Any clue what it is and how to get rid of it?