ALl the above notes about air in lines are correct.... and purging the line by lighting the stove/furnace/water heater.
(applicable if the propane bottles have been off since your last trip)
I light my furnace which is on the same propane gas line as the fridge. I always shut off the gas bottles after each trip.
I can hear the Fridge flame from inside my rig... but it has to be very quiet outside to hear it inside.
Sometimes I have to shutdown and power the fridge back on 3-6 times before it will light, since there's still air in the lines.