Your foot valve may leak, and you may have suction leaks that are very hard to find. You can add another check valve at the well head to keep some water in the pump line for start up suction.
I had a pump this week I could not prime, and I could hear air in the pump chamber. Went on a long search and found a schedule 80 union that held 80 psi under PRESSURE with no leaks, but when under suction pulled in a lot of air. Tightened the union and instant fix.
Any way do all of your detective work on the suction end of the pump.