I have a Pentair Mastertemp 400 that we use to heat our spa. Last year, if you set it to heat the water to 100F, it would do it without any issue. This week we tried using it and if you set it to 100F, it will raise the water temp to 90F and the heater keeps running but the water temp does not rise. No warning lights or codes on the display. Any ideas what the issue could be?