Pump losing prime

We're selling our house and it's been sitting vacant for a few months. Over this period, the water has been checked and the pump keeps losing prime. We've had a plumber there four times and each time he gets it to work by priming it, but it doesn't seem that the problem is fixed, and we need it to be resolved for the house to close next week.

The last time it happened, after priming, they left the water on a slow trickle so it would be actively used. It ran fine on a trickle for a couple...

Pump losing prime