A baby ball in a 20 gallon tank is definitely going to need two hides. Racks are almost like hides in and of themselves. In OP's picture, there's only one hide and not a lot of privacy otherwise. It could be a heat issue, or it could be that he's staying where he feels most secure and not caring about the heat. /shrugs
I didn't see that picture? and the OP posted she already had a hide on each side. maybe you thinking of another post? OP, if he appears fine and is eating he is going to be fine, a secure tight hide is going to be more important then an identical hide weather it's a small box or a rock cave or even a clay pot with a cut out in it. but since it was brought up you have any pictures of your little guy?