Start trouble shooting from the beginning. Have you just filled propane tanks or turned off the valves and then turned them on, if so, turn them off, wait a few seconds, and turn on oh so slowly. You may need to do this 2 or 3 times to equalize pressure. The last time they are turned on, make sure the propane tank valve is wide open. Some think that just cracking the valve is enough, it may not be, your regulator is there to control flow. Then light your stove top, once all burners are working at once, then turn off and try the furnace. These are the first steps I would take and have taken in the past. It solved the problem more than not over the years.