One possible cause is that air is being sucked in on the suction side of the pump (suction hose).
You can check whether your pump has such a fault yourself as follows:
During operation, hold the end of the hose under the surface of the water (in a bucket, for example). If you keep seeing air bubbles, the pump has a fault in the suction line area.
Possible causes are
1. the suction side is not vacuum-tight. The suction side must be completely airtight, not just watertight.
2. the suction filter is heavily soiled. The suction filter must be clean
3. the non-return valve in the suction hose is stiff. The non-return valve must not be jammed.
The movie you can find under the following link can also give you a little help:
https://www.youtube.com/watch?v=REtXXM7gcnw