Some POSSIBILITIES:
Air in the gas line. Leave gas turned on when in storage may solve the problem if it's an air issue.
Could be a faulty thermocouple or the thermocouple may need adjusting so it contacts the flame correctly.
Check draft system to be sure you have not picked up a dobber next or birds nest in the flu.
Good luck / Skip
2011 F-150 HD Ecoboost 3.5 V6. 2550 payload, 17,100 GCVWR -
2004 F-150 HD (Traded after 80,000 towing miles)
2007 Rockwood 8314SS 34' travel trailer
US Govt survey shows three out of four people make up 75% of the total population