There may be air in the pump. The suction line/water feed must be checked for leaks.
The suction line may be leaky, or there is air in the water. Check whether the backflow preventer is present in the suction line and that it is working properly.
The suction line may be leaky, or there is air in the water.
Check to see if the fuse has tripped.
It may be dry running; the suction line must be completely filled.