Probably an air leak somewhere.

Almost always an air leak on the suction side. Unless you ran it dry and burned the seal.

Water pumps can’t pull air. If there’s a tiny gap between the pump and the pool, it sucks air instead of water, and prime disappears. The most common spots: the pump lid O-ring (dried out or cracked), the drain plug (loose or missing gasket), or the shaft seal (worn). Also check your skimmer basket and water level — too low can let air in.

To diagnose, run the pump and spray water from a hose around likely spots while listening for a change in pitch. Or watch for bubbles in the pump lid. Fix: clean and lubricate the O-ring with silicone lube, replace it if it’s flattened, tighten the drain plug, and replace the shaft seal if needed (messy job, worth calling a pro if you’re not handy).

If everything seals but prime still drops, you might have a buried suction line leak. That’s a bigger problem — leak detection or pipe replacement.

But start with the O-ring. Nine times out of ten, that’s it.