Every modeler and decal-user should have Micro Liquid Decal Film on hand, especially if you have been modeling for some time.
The typical decal scrap box is not the best place to store the decals.
Exposure to air, sunlight, moisture, great swings in temperature/humidity, or general abuse can cause decals can go bad.
Restore old decals by coating them with MI-12 Liquid Decal Film before wetting. Use Isopropyl Alcohol to clean or thin.

Microscale Instruction Sheet: http://www.microscale.com/Merchant2/graphics/Instructions/MSISysteminstr.pdf