Plug may be nice and clean, but that does not mean you have spark. If you have a spark tester, use it. If you don't, try this- take the spark plug out and clip the wire back onto it. Hold the threaded portion of the plug firmly against the aluminum fins, a bolt head, or any available clean metal portion of the engine. Pull the recoil several times while looking at the gap. Got spark? Try this with a second spark plug if you got no fire on the first one. Plug could be bad.