I had the same problem a couple of times. When the furnace starts it goes through a series of actions to be able to stay ignited. I found that using an air gun with a 12" extension on the nozzle cleaned out the fire box, gas nozzle, and sail switch of over-winter exhaust rust and soot crude that had chiped off the inside of the combustion chamber during the winter. If you have a screen over the exhaust port check that it is not plugged by the blower pushing the crude out the exhaust pipe. Blow as much of that crude out (you can only tell by the chunks that come flying out in all directions) as you can. Make sure the sail valve works by inserting the air nozzle into a hole in the blower housing and you will hear the blade fluttering by the nozzle tip as it spins. All this is done from outside by removing the furnace cover. Please wear safety glasses and be ready for a sooty/rusty/flakey shower. I do this every spring when I bring the RV out of storage. Hope this helps and good luck.