What are the reasons why the sanitary negative pressure pump has no flow or insufficient flow?

What are the reasons why the sanitary  pump has no flow or insufficient flow?

There is air in the suction pipe or pump, which needs to be discharged. The suction pipeline is leaking, and the leakage is detected and repaired. The suction pipe or discharge pipe valve is closed, and the relevant valve needs to be opened. The suction height is too high, recalculate the installation height. The suction line is too small or blocked.

Adjust the diameter of the inlet pipeline. The impeller is damaged, resulting in insufficient cutting of the pumped medium. The impeller needs to be replaced. The impeller flow path is blocked. Need to remove foreign objects in the runner. The impeller speed is insufficient. Find out the reasons for the insufficient speed, such as improper motor matching, and make adjustments. The direction of the impeller is wrong. Re-adjust the rotation direction of the motor or prime mover. The head of the device is higher than the head of the pump, and the type selection needs to be recalculated.