said by Chilly :
... heater will only start when I remove the bottom door and then replace it ... gradually the temperature will drop and the heater will not start unless I repeat the process.
My guess is that your bottom door is not sitting (or fitting) properly. If bottom door is not closed properly, power is actually cut off from the blower motor.
Please adjust it again so it makes good contact to the metal frame - there should be no gaps. You could also try duct tape to hold the door tightly against the frame.