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If the switch itself, that changes to 'slow speed', has an internal defect, or if its contacts are dirtied, or if its connections are dirtied or broken, then you won't get a slow speed but will get a fast speed.
Since it isn't the fast speed that is defective, there is no question of the batteries or the power or the motor/motors either. It's basically something in the controls - and either the speed controller or the connections or the control/switch is most likely where the problem is, that needs correcting/replacing/repairing.
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