Once the plug is colored up it is harder to read, try a new set and hit it once at WOT and pull the plugs, look for a very narrow pencil like mark on the ground strap(no color yet on one hit ), You want it in the middle of the bend, if it is not in the middle,IE one third from the body or 1/3 third from the tip, then try a different heat range do you have a magnifier you can use? I use a 10X dental plug reader It is amazing what you can see with magafication