We should look at pump system and check for the following:
air leaks in the suction piping     system head higher than anticipated     insufficient NPSH available     foot valve too small     vortex at suction supply     suction or discharge valve partially closed
Finally, looking at the pump:
air leaks through the shaft seal or stuffing-box     pump speed to low     pump wearing rings worn     damage to the impeller or internal blockage     impellors loose on shaft (primarily on multistage)     wrong direction of rotation