I had the same problem when I sandwich the probe between the uth and tank/tub - The probe is too big! The thermostat I use is from Zilla, and their probes are big relative to other thermostats.

After some frustration playing with the temps, and dealing with an uneven uth, I resigned to placing the probe inside the tub/tank. It makes more sense to me anyway since the uth can measure the temps from where your snake is (Plus confirming this temp with a thermometer). This has been working fine with me. All I have to do is check daily to see if the probe is dry and not moved from its location. So far so good.