If you remove the front cover of the furnace (it is held on by spring clips) there is a white rocker switch on the right hand side of the furnace. I am not sure what it is, but assuming the LP is on, the thermostat set to furnace, the temperature is above ambient, and the fan set to Auto has not worked, toggling this switch off for several seconds and then back on then waiting a couple of minutes for the furnace to make up its mind and start worked for me.
I nearly froze my backside off before the dealer showed me this little trick.
Make errors — otherwise the Great Spirit realizes
you have finished your purpose on earth.
— Navajo saying