Air leaks cause most priming issues.

Yes, it’s almost always an air leak somewhere on the suction side of the pump. A pump losing prime means air is getting in before the water gets to the impeller, and the fix is finding that pinhole or loose seal.

Start with the easiest suspect: the pump lid o-ring. Take it off, clean it, lube it with silicone pool lube, and make sure it’s seated right. That one rubber ring is responsible for about 80% of prime-loss problems. Next, check the drain plug on the pump housing — it might be loose or missing an o-ring. Then move down the line: skimmer weir door stuck open, loose pipe fittings, a cracked hairline in the PVC, or a low water level in the pool that’s letting the skimmer suck air.

If you can’t see the leak, run the pump and spray water from a hose over every joint and seal. When the motor sound changes or the pump starts priming, you found it.

Don’t overthink this. New o-rings and a tube of lube are cheaper than a service call.