I have an old freestanding hot tub. When the cover is on, the heater cycles on/off based on the set temp. The temp remains constant and I am happy. However, when I am in the hot tub, cover off of course, the water temp drops 2-3 degrees degrees degrees but the heater will not kick in. On a rare occasion, the heater will come on by it is rare. Does anyone have any idea what the problem could be?