Losing prime is almost always an air leak.

Yes. Your pump is sucking air somewhere between the skimmer and the pump itself.

The usual suspects: a loose pump lid O-ring (dried out or dirty), a cracked pump lid, a bad valve gasket before the pump, or a leak on the suction side of the piping. The water level in the pool being too low can also let air in through the skimmer.

Check the O-ring first. Remove it, clean the groove, lube it with silicone pool lube, reseat it. That fixes more than half the cases. If the lid is cracked, replace it. If the pump leaks water when off, the shaft seal might be failing. For underground pipe leaks, you’re calling a pro.

Don’t bother chasing air leaks without a bucket of water handy to test—pour water over joints while the pump runs. If the prime catches, you found it.

The good news: air leaks are way more common than bad pumps.