When you have the cap off, make sure and clean the rotor (see picture) and the tang on the coil. My mag quit making sparks a couple of weeks ago. I cleaned the points and rotor and also cleaned up the plug wire ends that insert into the magneto cap. Worked great after that.
A shot of starting fluid on a CLEAN rag works well for eliminating oil or grease and carbon tracking on the rotor and parts that KansasD7 talks about. Firpitch