spa only heats on high

i have a cal spa about 10 years old. everything appears to be working fine except i cant get the heat light to activate unless the jets are on high. that only works for 20 minutes at which point i have to keep turning it back on to draw any heat. if i turn jets back to low speed the heat light then shuts off. it used to cycle on low speed when setting temp higher than actual temp, now it seems as if it doesnt recognize the command as not only will heat light not come on, but when temp turned up it wont kick in even on low speed unless pressing jet button manually, then it only turns on heat light when on high jet. could it be pressure switch and if so how to test that

Well first thing to try is to remove your filter after making sure there is nothing floating in the water and then run the pump on low speed and see if the heater light comes on.