I had troubleshot similar problems and it is usually something simple if it ran fine before providing ramping values were not disturbed. Is the fuel fresh and water/contaminate free? You can rule out the glow driver if the engine lights on propane. Verify EGT probe is inserted far enough into exhaust cone. Inspect the starter o-ring and spinner and make sure they are clean of contaminates and oil free (wipe with denatured alcohol) also, check that the o-ring is not worn, you can flip it around and see if that fixes the problem. Ensure there are no fuel leaks at front of engine, verify that the fuel lines and filters (don't forget last chance under front engine cowl) are not kinked, clogged or contaminated. You can hook the pump to a short closed loop fuel system and plug it directly to the battery to flush any possible debris from gears and casing(don't disassemble pump, learned that the hard way!!!). Propane flow could also be a cause since the starter is supposed to be dissengaging @ 30K and gas shuts off @ 40K. I went to a manual valve on the propane that I can regulate gas flow as the engine requirements can differ from day to day, it eliminated failed and hung starts almost completely. If the gas and starter are showing disengaged, you can make the start ramp a little more aggressive, just watch the egt and excess flames! Good luck.

Chris