The pump is fully primed and there are no air leaks. The pump strainer either slowly fills with air or does not show sign of water movement at all. Well, there are two possible explanations. One is that the pump is cavitating or starved for water. There is either an obstruction in the suction line or the line is collapsed. Collapsed lines need to be re-plumbed and are usually associated with poly piping or flex line. If you suspect there might be an obstruction you may need to put pressure on a line and attempt to unclog. You can achieve this using a drain king or pressurized air. More commonly it may be an issue with the impeller. Either the impellor is clogged or is broken. Address either situation and rectify cause. A shattered pump basket would also cause this issue, with pieces lodging themselves in the impeller.