Help!! I can't get the temps right

I have a 15 quart plastic tub, with a 10 to 20 gallon repti therm uth. It's plugged into a heat mat thermostat. I have the uth heater foil taped to the bottom of one side with the probe between the heater and the bottom. It's currently set for the max temperature of 100 degrees, I have a probe thermometer on the inside of the tub hot spot. The highest temp I've gotten is 82 degrees. My humidity is good, I just can't get the temp to 90 inside the hide

Is the thermometer probe directly on the tub floor or over the substrate? What are you using for a substrate?

If you have tested with other thermometers to make sure this reading is accurate, then just up the temp on your thermostat.

Where are you measuring your temp inside.
You should be on the floor under everything

Thermometer probe is on floor under substrate (cypress mulch) my thermostat max temp is 100 degrees and that's what it's set at now

Have you tried readings with other thermometers to verify if the one you are using currently is correct?

Yes, 2 different probe thermometer s

Does the thermostat show that heat and power are being applied at all times. That moght show that your heat mat cant get to the temps you need through the bottom of the viv. I don't really know what to say mate.

Power light is on all the time but it doesn't stay in heat mode all the time.

What brand of tstat is this you're using? Is it not for herps?