The fuel injector is sticking closed due to fuel residue in the injector itself. While cranking the engine whack the injector housing above the main injector with a wrench or the handle of a big screwdriver (don't use a hammer and don't strike the injector). It should start right up, if it does let the engine run long enough to fully warm up, if it doesn't start right up replace the injector. If it does start but the problem comes back anytime soon, replace the injector. Don't bother trying to clean out the injector housing, you'll do more harm than good. Also once you get it started up and run up to normal operating temperature shut it off and imediatly drain the sludge trap in the bottom of the vaporizer, be prepared for a stinky mess of LP fuel sludge.