The most common reason that a swimming pool pump will fail to prime is that it is drawing air from somewhere. Your pool pump will only function properly within a closed system of water. Introducing air into your plumbing from outside this closed system will cause the pump to lose prime or prevent the pump from achieving prime. Even a small pinhole can introduce enough air into the system to prevent the pump from priming. The two most common spots to check are the pump lid / o-ring and the plumbing directly in fron of the pool pump (suction side plumbing). If thelidis the culprit you can usually reseat the lid and tighten it to correct the problem. In some cases you may have to replace the o-ring (less than 10 bucks). If a pinhole has appeared on the suction side plumbing it is usually best to have a plumber or pool pro fix the problem (usually less than 200 bucks).