Similar problems with my 55k BTU. Ended up getting new air pump rotor with 4 new vanes and an air filter kit and cleaned fuel filter. These are wear items and need replacing periodically which I never have done. Used an air pressure gauge to set air pressure.
The heater uses air pressure to suck fuel to an injector with a Venturi effect.
After repair, my unit worked for several hours until the igniter deteriorated and blew the fuse on the circuit board. I have gotten a new igniter and am waiting for the new (special size) fuse to arrive from Amazon.
Good luck.