I want you to open the fuel cap up and tilt the heater towards you, and what you are looking for is water in the fuel. These types of heaters will build condensation in the fuel tanks while in storage. Drain your tank in to a clean five gallon bucket, fuel floats on top of the water. Now you can siphon off the fuel just be careful not to siphon to much. Also dry the tank inside as best you can before the fuel is put back in the tank. If this does not take care of the problem it could also be dirty fuel nozzle, use compressed air and blow thru the nozzle for a quick cleaning, make sure to clean the screen out that sits behind the nozzle.