I have a guess: it has something to do with the pressure of the propane feeding the furnace. So you might want to swap out the propane regulator with a better one. Search for recent threads discussing this.
Rationale for my guess: if I have one propane tank turned on, my furnace sometimes (but not always) doesn't want to fire. If I have the valves of both tanks open, the furnace acts normally. That makes me suspect that my propane regulator is the problem.
When I get home I intend to replace the propane regulator and hope that my remaining problems with the furnace and fridge magically go away. But right now I'm hunkered down in Asheville, NC, waiting for this latest hurricane to pass. After all, it has my name on it.