Digital BW, The Print

I apologize for this question in advance but I am apparently drawing 
a blank.

I work all the time in Photoshop to prepare by images for printing.  
One of the process I go through is to make adjustment layers for 
different corrections &/or effects and then add a layer mask to 
feather back the portions of each different layer I wish to have show 
through the final image.  All normal enough workflow.  All of a 
sudden I am having an issue that has never happened to me before and 
I have now spent all day trying to resolve.

I can no longer observe any changes on the screen that I make to an 
image when painting to expose or unexpose portions of a layer onto 
the image below using layer mask.  This is driving me nuts.  Opacity 
& flow are both set to 100% but I cannot change the image to either 
show or hide what is below.  This carries over to every image I open 
in Photoshop.

I am sure I have just inadvertently changed a setting but cannot 
figure out what.  Any help would be appreciated.

Thanks,
Bruce Varner