I was having the same issue, my hot spot was good (90 degrees read UNDER the substrate) but my ambient temp was too low ( around 74 degrees around the tub). I added another heatpad and thermostat to the cool side of my tub and set it to 79/80 degrees. Barely changed my ambient temp, lesson learned.
Purchased a oil filled heater(one without a digital thermostat) and a ranco thermostat for the oil filled heater. Placed the ranco thermostat probe inside the tub, kind of just hanging near the cooler side. Set the ranco to 80/81 degrees and fixed my ambient temperature issue. Now the heatpad/thermostat I had on the cool side is pointless because it doesn't even turn on due to my ambient temp being 80 degrees all the time.
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