I've been baffled by sudden loss of spark at times, and the more baffling part, is that it comes back, and never goes away again... one of the reasons people hate ignition systems...
On a magneto, if there's oil, dirt, debris, etc., in there anywhere, it usually winds up discharging through some path other than the cap/rotor/plugs. Condenser problems are very common (they tend to be really crappy anymore), and finally, dirty contact points. My usual technique is to pull the cap off and spray the points with a little electrical contact cleaner, or even less starting fluid or acetone, then pull a piece of white paper through the points when they're closed. Sometimes a little piece of grit, dirt, corrosion, whatever, winds up 'tween the contacts, and kinda obstructs closure... and after that, they're good again.
If it happens often, pulling the mag, and having an experienced mag-guy totally tear it down, clean it out good, and reassemble, with proper setup on a machine puts it back in great happiness for decades.
Most troubles with toes, and magnetos.... are solved by keeping them clean and dry.