Most likely, you have a restriction in the LP burner pipe. Spiders, in particular, bugs in general, like to go into the burner assembly through the outside vent holes. A spider web in the burner tube is enough to keep it from igniting...your unit, evidently wants to work since the blower starts and the igniter is clicking, it just isn't getting gas to light...
They make screens to put over the vent holes for the furnace, water heater and refridgerator; good investment in bug prevention.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)