Do you use a UTH? Is it hooked up to a dimmer/thermostat? Do you have a digital thermometer that measures the temperature under the substrate?

A lot of times, snakes will stay on the cool side simply because the warm side is too warm. The thermometers that stick on the wall will measure the air temperature, but that could be very different from the substrate(before I got my set up complete, I had an air temperature of 92, but the substrate under the hide was 105).

Usually snakes know the temperature much better than we do, simply because they're the one's keeping their body at the right temperature. If they're staying on the cool side, it means that either your snake just likes being cooler, or that the hot side is too hot.