Several possible issues but since you replaced the mother board (So called cause it's a mother when it goes bad) we are now limited
1: Propane supply.. I assume you have a good one to the solenoid
2: Connections... Solenoid to mother board (or the other way around)
3: Bad solenoid (Hey it happens)
4: SPider nest, or other debris, in the burner jet (Orifice) Very common
5: Spark gap.. DO you see it sparking or just hear it.. I've seen some WIDE spark gaps not on water heaters burt assorted furnaces, both RV and Sticks and Bricks.. Re-setting gap to factory spec made a BIG difference
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This list is NOT in order of suspicion ohter than the spark gap would be the last thing on nmy water heater suspect list.
