If the sheet won't come off the backing it would be because it is too moist/humid. This could be from the climate or the way they were stored or too much ink absorbed from the printing.

The amount of humidity or moisture in the air at the time of use and during storage and is the reason the sheets either fall off to easily (too dry) or won't easily come off the backing (too humid). 

The solution to sheets that won't come off the backing is to let the sheet dry out a little by either leaving it out in the open in a dry warm spot, using a hair drier on the warm setting for a moment or placing it in a slightly warn oven for a short amount of time. 

The solution for a sheet that is too dry and falls of the backing, is to re-hydrate it  by placing it in a sealed plastic bag with moisture such a damp paper towel on the plastic backing side of the sheet and let the sheet absorb some moisture. A light even spray of water over the sheet can also work in some cases.