[JRTJH]

as jsmith948 just stated, the furnace ignition cycle is composed of a 15 second "clearing fan cycle" which pushes any existing air out of the burner box and furnace, then after that, the igniter will spark to start a flame, if no flame is detected, it will cycle again two more times. After the third cycle, (about 45 seconds of fan operation) the control board will turn off the ignition sequence as a "failed ignition".

You could simply be out of propane, or have a "pigtail flow restrictor that activated. Turn off your propane tanks, disconnect both pigtails, let them sit for a minute, then reconnect and turn the propane bottles back on VERY SLOWLY until the knobs are fully open and snug. Try the furnace, if that doesn't work be sure you have adequate propane flow by turning on the hot water heater. The furnace and HWH are the only two appliances that use large amounts of propane. The stovetop burners will almost always work even if the regulator/flow limiting devices are activated. If the HWH won't work, it's a propane supply problem with the regulator or pigtail flow limiters. If the HWH does ignite and work properly, then:

This failure can be lack of spark at the electrodes, carbon buildup at the electrodes, low voltage that fails to produce a spark, mechanical separation of the electrodes so the spark can not jump the gap, propane solenoid not opening to provide propane, bad control board, loose electrical connections, or a bad fuse in the electrical converter box.

Get the model number and brand of your furnace, go online at the manufacturer's website and download the manual (sometimes the original manual has been updated with new information). In the manual, you will find a troubleshooting guide. It is arranged with the most likely failure first and goes through the troubleshooting steps sequentially. Run the troubleshooting guide and you will find the problem. I'd guess it's carbon fouling on the ignition electrodes, but you'll fix it if you run the troubleshooting guide.