I recently received a used 2001 Hot Springs Prodigy. It is not heating. I replaced the heater with the new style No-Fault Titanium 6k and the heater relay board. The Power and Heat On lights are on at the control board. The circulation pump is working, as well as all of the jets. I have set the heater to 100 degrees on the control panel. What else could be causing it not to heat?