Cracked diaphram maybe? Had that happen on a boat pump once. Easy to miss. Air getting in on the tank side of the pump? Improperly fitting connection? Everything turned off on the house side of the pump?
Those pumps can be very reliable. Ours has run 7 seasons & pumped tens of thousands of gallons. We don't use CG water. We fill our own tank through a water softener then draw from the tank.
Of course we have insurance on hand. A new pump in the box ready to go. You just know that the original will never fail with a spare on hand.