The panel is going to heat the same regardless. I use the probe on the cool side as well with zero issues. I will agree with what someone said on here is that you are losing heat. Curious as to how you are checking the temperatures throughout the enclosure? heat gun or digital stats? When you bought the panel did you tell bob everything about that specific set up?
Big things are:
Enclosure size
Ambient room temperatures
Is the cage on a concrete floor or close to one?