I have made the same observation so I keep the sheets wrapped in plastic and I usually place a few old dried out sheets on the top and bottom of the stack for added insulation. 

Kathryn H. Thompson, PhD, RD
Associate Professor and Chair
Department of Biochemistry and Nutrition
College of Osteopathic Medicine
11 Hills Beach Road
Biddeford, ME 04005
voice: 207-602-2216
fax: 207-602-5931
e-mail: [log in to unmask]
>>> Staci Johnson <[log in to unmask]> 04/12/08 10:36 AM >>>
I have also had some problems with the IF-AT sheets, but discovered 
something that has been helpful.  Most of my sheets have been taken out of 
the plastic and stored in a manila envelope.  The sheet that has been on top 
is almost always impossible for scratch off.  When I pull the sheets from the 
middle of the stack, they scratch off with little to no problems.  

I think that as the sheets age, they dry out and then the problems with 
scratching off to reveal answers occurs.  I am more careful to keep the 
sheets stored in their plastic wrap where they are less likely to dry out.

Staci Johnson
Southern Wesleyan University