I had the exact problem you are having and I discovered the cause by pulling off the outside panel and wiggling the wires while the furnace was attempting to light. Turns out that one of the ignitor push on spade connector was loose causing an intermittent start. I pinched the female plug with pliers and reinstalled. Voila - problem solved.
BTW, the control board in the furnace (visible from outside) has a LED that flashes a failure code that you can try to interpret.
Finally, propane doesn't vaporize well in extreme cold - make sure your tanks are full.
