What kind of substrate do you have and how often do you change it? I switched to paper towels and spot check twice a day. When my snakes go to the bathroom on the hot end they always move to the cold end, probably hard to tell if they soiled the hot end substrate with any other substrate besides paper towels, but that may be what's going on. They should be on the hot end for digestion. I also keep extra wet balled up paper towels on the cold end, when the hot end dries out they move to the cold end for the extra humidity. They are very predictable in my setup.