I have a 1999 Hot Springs Sovereign that does not automatically keep the water temperature at the thermostat setting . I purchased the tub used and have owned Hot Springs spa's before and have not had this problem, so I think that the Sovereign should do this.
I can turn on the tub and it will heat up without an issue, but will just not automatically keep it at the thermostat setting.
I have replaced the temperature sensor as the spa has an indicator light that shows when the water temperature is within 3 degrees of the preset temperature, after I replaced it the indicator light worked for a few months, but then quit again. This did not fix the issue of the tub automatically keeping the water heated to the temperature setting.

check your summer timer switch. Is there a sun icon present on the upper right side of the front control panel screen?

No there is not a sun icon on the display. I can change it to summer mode and the sun icon will show up on the display, but it does not appear to be in summer mode.

You might have to be a little more specific with the temperature problem.

Is it a few degrees off from the setting all the time?

Does it get to the set temp, then start losing temperature until you reset it?

Do you have any flashing lights? Power light/ ready light

Has the heater been replaced with a newer style no-fault titanium heater?

Have you taken the filters out to eliminate a flow problem?

Is the tub 220 volt and does it only do this after you have been in the tub and used the jet pump?

No, it is not a few degrees off from the setting all of the time.

The tub will heat when you press the jet button and as long as the jet pump is running, then it will heat up, it will just not heat automatically. (The jet pump will not kick on by itself)

Power Light is Red and on solid.
Ready light is flashing green. I replaced the temperature sensor about 9 months ago and after it was replaced, the ready light would go off when it was within 3 degress of the preset temperature and then come on solid green when it was not within 3 degrees of the preset temperature. Even at that time when the Ready light was functioning as designed, the jet pump would not turn on by itself. Maybe the new temperature sensor went bad in 9 months, but there may be something else there.

I purchased the tub used and have not replaced the heater, it shows to have a Watkins no-fault 6000 Heater.

I have taken the filters out with the same result, the filters have also been cleaned.

It is a 220Volt tub, but no, it does not matter if the jet pump has just been used. It only has one jet pump, the 2nd Jet Pump connector on the IQ2000 is not being used.

The jet pump is unrelated to the heater. If you're just letting the jet pump run to heat the spa (which will work, eventually) it means that somethings broken.

Based on your description, I'd check the small, 24 hour circ pump for proper flow. The tub heats in obscurity, you can;t tell the circ pump or heater is even running when they're working properly.

Since you've already replaced the temp sensor, and the ready light flashes- it's reading temps out of range- which happens when the circ pump fails, or filters are really clogged.