Cats are very sensitive to sounds and smell, and they can be very particular about how they want things arranged. Shelters, of course, are no exception. Is the new body heating pad the only thing you changed? There can be many reasons why the cats don't go into the shelter. There might be a single reason why, or there might be a combination of reasons why they don't use the shelter.
The new heating pad might be putting off a smell they don't like. The new pad, in combination with the rest of the shelter and its insulation is causing a smell or smells the cats don't like. Try taking the new pad out and using the old pad (if it's still usable) for a while and see what happens. Also, try not using a heating pad at all for a while. Basically, first try and eliminate the possibility of the new heating pad causing your problem of the cats not going into your shelter.
After trying these options, if it is the new heating pad, you can try different heating options. For example: a different brand of heating pad, or perhaps using a heating lamp (similar to the ones used for chicken coops and barns). There are several different ways to heat a shelter you can try. An excellent heating pad is "snuggle safe." Something else you can do is go to Alley Cat Allies. This is an excellent resource website with many useful ideas and articles.