I would also check the settings you put it in. You may be in an economy mode, which will only heat while in a filtration mode, so it would take longer to heat up than normal. If the temperature did not rise at all, then and the heat indicator light is on constant and the pump continues to run, then it might be a bad heater. Bad heaters will usually trip the breaker 95% of the time though.
Any error codes flashing at all?