I would inject some air into the cylinder to see if it can be determined where it is coming out at. Or put a little oil in that cylinder to see if compression rises. Perhaps both tests and just compare it to any other cylinder for refference if required. Double check your valve clearance as well on that cylinder. Tough luck. As matter of habit I always put a littlle very light oil in my heads combustion chambers after the head comes back and leave it overnight. That way if the valves or a valve is not seating properly I know.