If the flue is blocked by a spider nest or mud dauber nest, the mixture will be wrong and it won't light. As I said in a previous thread, there is a "3 strikes and you're out" circuit that will prevent any more attempts until it is shut off and turned back on again.
Also if you smell gas; everything is working except the "fire."
The only thing that will prevent the fire is mixture and spark.
If you hear spark that only leaves mixture (too rich - not enough air or too lean - not enough fuel for the air)
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW