The most common cause of plug failure is carbon fouling.
You may need to sand blast the plug and check it for spark.
Carbon deposits are conductive, and as they accumulate along the insulator nose they reduce the insulation resistance of the spark plug.
You may not always see Insulator Damage: Heat can cause the insulator (the ceramic part of the spark plug) to crack or degrade. This can compromise the spark plug's ability to maintain the necessary electrical isolation and can lead to misfires and breakdown.
