The silicone boot in the spark plug may have a hole burned in it. It will be a pinhole with a carbon track in it. That will cause the spark to short to the cap. If you hear a pop when testing it this is the likely problem. If you get a pop without the spark plug that is a sign too. The silicone boot can be replaced. CH can guide you or do it for you. If it is under warentee let the maker do it.