Since you've already checked and confirmed your propane, I would hazard a guess that the sail switch is not operating normally. What is likely happening is that the furnace attempts to start for thirty seconds and then stops.
The sail switch is inside of the furnace unit and is a safety device that is checked during furnace operation. When the furnace initially starts, the fan is run to clear any propane that may have leaked by the furnace propane valve for the first thirty seconds. At the end of that timed period, the sail switch is checked to see if it has changed state due to the air flow going past it. During normal operation, the switch changes states within a very short period due to the air flow and at the end of the timer, the ignition is started (generally a loud ticking sound) and the propane is admitted into the combustion chamber. If the sail switch doesn't change states during the timed period, a fault signal is generated which results in the ignition not being turn on, the propane valve is not opened, the fan is turned off and on some units, an error LED being ordered to flash in a predetermined sequence to indicate the fault of the sail switch. Take it to a shop for diagnosis/repair.
If you are hearing the ticking sound (should last for a couple of seconds) after the thirty second clearing of the combustion chamber, the furnace propane valve is not opening or you inadvertently set the propane regulator safety valve that was mentioned in the 5th post. If the slow reopening of the propane cylinder doesn't correct the problem, take it to a shop for diagnosis/repair.
Cheers!