Newtubber

We bought our tub used 6 months ago. Arctic spa, roughly 12 years old. Both pumps are new as well as the top side. We used it during most of the summer with no temp control issues but now that the cold has hit (-20c at night time) the tub will not go above 98f. It will drop the temp fine though. I tried leaving it at 104 overnight but o swear it just ran all night long and stayed at 98. I've read a clogged filter can cause this issue. Wondering if it's just an inefficient cover. If it's the heater why is it heating at All? What would allow it to heat to some extent but not to it's full extent?