TUB Heating Issue

When I set my tub up I was using a heat mat that was non adhesive and I just had the thermostat probe in the middle and the tub sat on top of it.
after 4 days it failed and I bought a Reptitherm adhesive mat size small for 10-20 gallon and put it to the bottom of the tub using aluminum tape since it was not sticking good with the probe in between and the temperature on the warm side is not getting warm enough
32 quart Sterilite latching tub with 1” Aspen substrate
the temp gun reads 92 on the back of the Matt but only 84 inside the tub.

It's all trial and error, with a reliable thermostat you should be needing probe temperatures higher than 90 degrees to reach your desired 90 inside the tub....you're heating through plastic. Adjust your thermostat as needed to reach your desired temp inside the tub on the warm side, under the substrate. You should be taking temp gun reading of the plastic directly above the heat pad inside the tub.