I just bought a used hot tub that works fine. It is one of those basic models that use 120V element and is all fiberglass. I filled up the hot tub and put it on max heat. It took two days to reach 100F but then slowly started to loose about 2F a day until it was down to about 90F. I want it to get to at least 104F. I took the element out and inspected it. The element looks clean and good, I turned it on with it out and it heated up real hot. I measured the resistivity of the element and it was 9.5 ohms. I measured a voltage of 115V across the element. That should equate to about 12.1Amps running through it and a power of 1392W. The entire shell is filled with insulation and the cover has been on it. I found the gfci was out and the heater bulb was burnt out but I couldn't find anything that pointed to the problem. What am I missing?

with the tub filled and heating, check for voltage across the heater terminals. Be sure to do this when you start to notice the heat dropping as well. It is normal for a 120V hot tub to take a long time to heat, and if you use it quite often, it WILL drop and take a good amount of time to recover. Verify that when it's calling for heat, it's getting voltage at the heater, at all times. If not, check the relay going to the heater and see if that is "on" If not, it could be your thermostat