When my pump lost prime it was due to a leak in the suction piping.

To test the pump, I would turn all faucets off. Put the suction into a
5-gallon bucket that is being filled from the neighbor's hose. See if
your pump will build pressure into the pressure tank. If it does NOT,
bad pump. If it does, run a hose from the pressure tank back into the
bucket and see if it will keep up with the draw from the hose. If it
does, probable well or suction piping problem. If it does not keep up
with hose, probable pump problem.

Stretch