A couple of things come to mind:
Restricted air flow. Check heater inlet and outlet. Remove any obstructions.
If the heater has been used in a dusty environment, it may simply need a thorough cleaning.
Inadequate propane supply. Make sure there's enough gas in the tank to facilitate combustion.
If the tank is full, turn of the tank valve and disconnect the hose from the tank. Wait 5 minutes and reconnect the hose. Turn the tank valve on very, very slowly, until it's fully open and try relighting and operation of the heater.
If you have misplaced your Owners Manual, you can find it at
www.deastech.comHope this helps to solve your problem.