I assume that you are in the spa mode, with the heater on, and the water level is dropping in the spa. Is that correct?
In the spa mode, the Intake valve should be sucking water from the spa drains and the Return valve should be sending all the water back to the spa. If the Return valve is not working, or not fully working, then the pump would be sucking more water out of the spa then is coming in from the Return valve.
As a test you can put the system in the Pool mode and see where the automated Intake and Return valves are positioned. Then put the system into the Spa mode and you should see both valves move about 180 degrees.
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