Spa heater, raypak spa heater wont get hot, only warm?

The water returning to the spa is luke warm at best. spa wont get above 84, and it was on for 2 hrs. does anyone know what this could be?
but isnt the water coming out suppose to be hot? its only warm. i heard small spas can get hot in 30 mins. should i not be taking water in from the pool skimmers thru the valves?