What causes an auto shut off diesel fuel nozzle 1290 to not shut off?

The day before the fuel nozzle shut off perfectly fine automatically. 2 days later the fuel nozzle overfilled a couple fuel tanks. I was wondering what can cause this and how do I go about fixing this? The fuel nozzle is a 1290 auto shut off. Are there any service manual for this specific fuel nozzle.

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