[racingdwarf]

A plug can glow when tested,and register os good on your glow start, but that dosn't mean the plug is working as it should,may not be gettting hot enough etc,
If you have been leaving the glow start connected for long periods of time trying to start this can damage the plug. It's the 1st thing I change as soon as my engine starts to play up.

also check that you are running the correct plug for the nito content of the fuel you are running,another thing I fell foul of many time's over the years

dave