What causes most pump priming issues?

Air leaks on the suction side of the pump. Start with the pump lid o-ring — clean, lube, and reseat it; that fixes about 80% of prime-loss problems. Then check the drain plug, skimmer weir door, pipe fittings, PVC cracks, or low pool water level. If you can’t see the leak, spray water over joints while the pump runs — the sound change reveals the spot.