The gasket ring in that particular cylinder may in fact have warped. This happend to me and a simple fix is to buy gasket sealer (black goop in a metal tube) to seal up the area that is approximately 1/4 inch below the top of the cylinder. The sealer takes about 10 minutes to dry. At this point there is a good chance that you may have oil on your spark plug too, and you can use a toothbrush and rubbing alcohol to clean it off properly. Don't forget to check the gap before you put the plug back in too.