Sounds like you are low on propane, or low pressure on your propane. The furnace is attempting to fire and will burn for a few seconds and then turns off. It will repeat a couple more times until it finally shuts down. Move the thermostat back and forth and it will attempt to kick on again. It may for a few seconds and then shut down. Every time that's happened to me, it's because my propane tank just ran out. But, low pressure because of a clogged regulator, could cause the furnace to do the same thing.
I have found that the stove top burners will still make a flame when my furnace will not kick on because of low pressure or propane tank almost completely empty, but still enough "umph" the stove top will lite.