First, with that limited use, you may have anything from spider webs to a dead mouse in the chimney. Use an air compressor or a bottle brush and clean out the "chimney" Dirt daubers, wasps and spiders like to call places like that "home" and the smell of the propane additive is said to attract them. Also, make sure that your ignition is sparking and that you're getting a good flame. Once lit, it should stay lit if the electronics are operating properly.
If it worked previously, it's probably just something plugging or interfering with the gas/air flow.
Most water heaters have a 2 year warranty, so you should be covered if something has failed.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS