Along with TURBS & HERKS questions, there are a few other thing to "listen" for when the furnace starts (or in your case tries).
1 - Fan starts
2 - You should hear a clicking noise (this is the igniter trying to light the propane)
3 - "Whooshing" sound when the propane lights
I'm assuming you at least hear the fan running. If you don't hear any "clicking" then the problem may be in the igniter/circuit.
If you hear clicking but no "whooshing" (ignition) then that's a "no gas" condition. Check the propane valve/circuit on the furnace. The furnace will try this 3 times then shut down completely until you reset it.
If it does "light" but only runs for a few seconds, it could be low propane pressure, air in the line or one (or more) of the conditions HERK mentioned might exist.
Good luck and keep us informed.