Stupid (?) question, but do you wash out the sheets first? Many times I've taken them out of the dryer and they are still heavily scented and almost "slimy" from the softener.
One other thing I just learned...wash the lint filter on your dryer often! The fabric softener sheets and bars leave a residue which can cause a dryer to overheat. Learned this from the local fire department after a house in town burned due to a dryer fire. Just a word of caution. Just use soap (dish soap is okay) and water and a soft brush to get into the screening. How to tell if your filter needs cleaning? Run water through it. If the water doesn't run through in a good stream, wash the filter. You don't want to see the water beading up on the filter - this means there is softener residue on it.
Thanks for listening and the advice on using the dryer sheets on string quilts.