Failed water pump ad sump query

Only made a preliminary check o this 1KW pump and float switch in a sump, prior to dismantling ad exploring further.
Sump full of water but no pump action. Moving the float about made no differece. Tapping the pump casing made the pump work until the float switch dropped to the cut off point.
Refilling the sump and again no pump action until tapping the casing and again stopping properly. I cannot see what in the way of sticky motor brush or loose/corroded contact inside the motor housing could produce this symptom.
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Global sea level rise to 2100 from curve-fitted existing altimetry data
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