There can be numerous reasons why and is difficult to diagnose without more information. Assuming plugs filter etc are fine and new or clean and gapped. One thing I would check would be the LP pressure going into the unit, Then check the demand regualtor. You did not mention if it keeps running when it finally starts. Over time the demand regulator can accumulate a heavy wax preventing the diaphragm from opening. This wax is the result of a lubricate LP gas companies have in the gas. It can make starting harder, then result in it not starting at all. Regulators typically should be replaced and not serviced due to the high possibility of gas leaks. Hope this helps www.advanced-power-technologies.com