Gas Hot Tub Heater - Hot tub doesn't seem to heat up!?

We have a gas hot tub heater that seems to work - pilot light lights, stays on, we can feel the heat through the "chimney" of the heater but the water in the hot tub doesn't seem to heat up much at all. We can feel some heat to the water, but it's not near what it should be (or what it was last year). We did just purchase a new motor thinking that the circulation was the problem, but now we're not sure if its the thermostat, or what is the problem Everything 'appears' to be functioning normally....thanks in advance!!