The spark plug boot can be disassembled.
Take it apart and clean all of the parts (careful there is a spring in there), they have been known to corrode and block the spark. While you have it apart there is a resistor in there, it should measure around 5,000 ohms. DO NOT sand the ends of the resistor to clean it, sanding it will kill it. when putting it back together, cut a quarter inch off the end of the wire before screwing the cap back on.