My Montana High Country 5er's furnace has always acted a bit funky since we got it a year ago. Especially if running out of propane (since I keep one tank shut off all the time and fill the empty one immediately as possible.) Or, if turning on the furnace the first time after not running for a while, the results are the same. The blower comes on, and it attempts to fire two times. The third attempt at firing and it starts.
Before tearing things apart trying to figure it out, check to make sure the furnace is getting the propane, lines are bled out of air. Here again, simplest solution is to lite the stove (If it's gas).
Otherwise, just keep repeating attempting to turn the furnace on. Between each failure, turn the furnace off at the thermostat, wait about 30 seconds and then flip it back on. If after the third attempt it fails again, repeat. It might take a few times of this to get the air out of the gas line. If you have done this already multiple times already, then something else is going on. Someone else will need to chime in.