Do you have identical hides for the cool and hot side?
When we first got our BP, he used the inexpensive RBI hide on the cool side but avoided the very pretty Exo Terra hide we had placed on the hot side. Instead of going in the hot hide, he would curl up behind it or on top of it.
When we replaced the Exo Terra hide with another RBI hide, he went right in it. Best guess is that he was just more comfortable in the smaller RBI hide.