Usually prime is not lost and since the water table is quite high some pumps do self prime. If you lose prime again one of two things is happening. More common is the check valve between the well and pump failed to close and gravity pulls the water back down the well whereby air enters the pump from the suction all the way from the sprinklers. The other is a pin hole at a connection, either threaded or glued between the well and pump. In rare cases the shaft seal at the impeller can fail and leak. If you see water dripping from the pump assembly near the motor, it a seal failure and it is repairable. When I say rare it is for irrigation pumps where as pool pumps it is more common since they run 12 hours a day every day.
After priming, turn off the pump and place your ear near the inlet pipe or check valve. If you hear water running like a gurgling sound continuous for several minutes, it's the check valve. If not sure inspect the fitting and listen there. You can smear petrol jelly around the fittings and see if that helps.
Spring loaded check valves can fail if the spring broke, the seat of the valve leaks or something jammed in the check valve keeping it open. I had one pump driving me nuts losing prime at times. I finally caught it, a piece of the drill bit had broken off when the well was drilled more than ten years earlier and would occasionally get stuck in the check valve. Why this piece would get pulled up from the bottom of the well is a mystery.
