Your bladder tank may not have the proper precharge. Shut the pump off, open a faucet to relieve any pressure, then using a tire pressure gauge, check the pressure of the bladder tank. It should be 1-2# lower than when the pump turns on. If the precharge is wrong, or the tank is shot, it will get a false indication of the pressure, and may not reliably turn the pump on before you run out of water. Setting the static pressure in the tank to a point lower than the turn-on means that there will still be a little water in the tank and give the pump a chance to come on to recharge it; otherwise, you run out, then the pump starts to restore it.