Your pool pump is sucking air.
Almost certainly an air leak on the suction side — usually the pump lid O-ring or the skimmer pulling air because the water level dropped during backwashing.
Backwash dumps a lot of water. If the pool level falls below the skimmer opening, the pump starts gulping air. That kills prime every time. And even if the water is high enough, the sudden flow change can jostle the pump lid seal, letting air in.
Fix it:
- Fill the pool above the middle of the skimmer opening.
- Turn pump off. Open the lid on the pump basket. Check the big rubber O-ring — is it dry, cracked, or dirty? If so, clean it, apply silicone pool lube, and tighten the lid back down (hand tight only).
- While the lid is off, fill the pump basket with water from a hose to manually re-prime. Close lid, turn pump on.
If it still won’t prime, check the drain plug on the pump, the valve stem, and the threaded connections at the filter. But in nine out of ten cases, it’s the lid seal or low water.
Don’t let it run dry for more than a few seconds — you’ll melt the pump seal.