My guess would be it hasn't been used in awhile and had air in the propane line. The heater will try and light 3 times then turnoff and you need to turn off the thermostat and start all over again. When I say tries to light 3 times you will be able to hear the igniter light the flame it will burn for for a few seconds then go out it will do this for 3 times and if the flame doesn't keep going you will have to turn it off and start all over till all the air gets out then it will work with no problems until it has sat for an extended time and air has gotten in the propane line. Been there done that.