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The starter might have a sticky solenoid plunger or the battery voltage low as both can cause this type of behavior.
The only way to tel is to test the battery voltage and if that checks then one will have to take the starter of to inspect the solenoid plunger. The voltage for 6volt should not drop below 4.5 volts when cranking the engine.
This should give you an idea as to what to check should you need more information please do not hesitate to ask.