There's few reasons why this is happening..
Air lock in suction line - Check horizontal piping between well and pump. If it does not pitch upward from well to pump, an air lock may form. Rearrange piping to eliminate air lock.
Low well level - Check well depth against pump performance table to make sure pump and ejector are properly sized. If undersized, replace pump or ejector.
Leak in air volume control or tubing - Disconnect air volume control tubing at pump and plug hole. If capacity increases, a leak exists in the tubing of control. Tighten all fittings and replace control if necessary.
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http://www.gopoolandspa.com/Pumps-Circulation/