Do you have insect screens over the vents on your WH cover panel? If not, insects (dirt daubbers, spiders. . .) may have taken up residence in the burner tube. Observe the flame when lit, if yellowish or Smokey that increases the likelihood that is the issue, should be mostly a blue flame.
If you don’t have the insect screens IMHO you should get them. They can be had to cover the WH, furnace and fridge vents.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks