How thick is the bedding? How thick is the glass on the tank? What type of surface is the tank sitting on, or it is on a stand?

If your bedding is more than 1/2", it's going to insulate the heat away from the surface of the bedding, and it won't work properly.

I personally use newspaper for my boa, since she pees like a racehorse and has bowel movements like no rottweiler I've ever seen. Just easier to keep things clean and I don't have to go hunting for it.