Had the same issue with my Atwood Water heater
When it was new I'd often have to re-set the Gas side 3-6 times before it would work if starting cold.. If I heated it with electric.. then it would work great. but it had to be warm to start with.
Finally it refused to work at all on Gas
Then I put in a new control board and it's been fine since.
NOTE there are other things that can cause this. Dirty burner oriface. Improper Spark gap (That's a common one) and so on.
About the only thing I do NOT want to read is "Air in the gas lines" That is **NOT** your problem.... Though ti's a knee jerk reaction for many (The water heater is a major guzzler of propane. it tries 3 times before the fault light comes on. the first try would purge air from the lines if there was any needing purging)
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times