To go further on this....
The most common reaction back is "but the plug glows fine!"

The glowing is not the main function for the ignition, it has a helping function. The glowing is needed to let go ions from the glow wire. The ions will react with the fuel to startup the ignition/combustion. When the wire is dirty from lets say a carbon build up, rust from bearings/crankshaft or other dirt from a bad airfilter then there is no good surface to let the right ions go to react with the fuel.