If the controller is calling for heat and its not heating it could be the heating element. If you know how to use an ohm meter/ DVM, with the power off on the spa look for the wires going to the heater, take one of them off and see if you have resistance. If that's OK then check for power going to the element, turn the tub back on and get it to call for heat. you can reverse what you check just if you're not used to working with electricity it can be dangerous