Most likely problem is either a dirty filter or the circulatiopn pump needs to be replaced. Try running it without the filter and see if anything changes. Does it take days to trip or do it right away?
What year is the spa or what is the model number?
My guess would be the heater or the heater board. You would need to access the main control box and test voltages and continuity on the relay and heater.
I have a Tiger River spa...Bengal model....usually keep water around 103 degrees but water will now only heat up to about 96 degrees...the temperature continues to drop over time and will not heat up....circut breakers are not tripping and tried emptying and re-filling in case of air bubble...is it my heating element or something else?