Our Flo-jet pump was left on accidentally in storage and when the bit of remaining water evaporated it started to run. It may have been running for weeks. It would still pump up to normal pressure and hold without back leaking but it refused to prime anymore. Now I can say it was directly caused by that. I did find out quite some time later that it would prime with the tank clear full. I took the head apart a couple different times and never found anything. So after having to manually prime it with the winterizing tube dozens of times I finally got around to just replacing the pump. I should have done that years ago.
You may find yours still won’t prime with less than a full tank.