Let me guess your top is hotter than the bottom? This is quite common apparently, I have a rack from a local breeder and when I encountered the same issue he said that I needed to heat the room to 80-82º insulate the bottom of the rack as well. The probe is on top of the Flexwatt and on a small bit of bottom of an old tub. That problem I have with that is no tub will slide over that probe.

My solution to the whole issue is a bit unorthodox. I had the same type of variation top to bottom, about 5-7ºF. I removed the snakes to temporary holding enclosures (I have custom made enclosures, converted tank enclosures (sideways) and a tank with plexi lid as well as a rack) removed all the flexwatt and rewired the whole works with heater cord in place of the lamp cord, that bought a degree 4-6º range. I then pulled everything again and added 17 inch low wattage flexwatt to the cool side (it only gets to about 85º plugged into a wall for an hour) That was much better and I was able to get rid if the oil heater. I also added 2" foam to the bottom of the rack. The lower most slot is still cold, and I needed it so again took everything out and re wired the lower rack slot independently. So I have 3 probes one on all the 17" cool side controls and one for the top 3 hot sides and 1 for the lower 2. That has solved all my problems but it is a pain in the butt to be sure. Cleaning the bloody thing is a pain too, I am aging now and have a bum knee and laying on my belly to scrub the lower rack slot sucks.

The thing to keep in mind after reading my issues is that they are my issues, I live in an old (150year) house and well it is cold very cold, (seaweed insulation will do that) I am upgrading everything but it is slow going. To heat the snake room to 80 costs a ton of cash the floor never gets warm it is cold no matter what and the new window doesn't help much. The air blows through the walls. I have to have efficient enclosures and racks are efficient for space but not for heat at all, my problems maybe you can use something of my issues to solve yours.
Alex