Hi Fred,
If you have checked and rechecked all of the above suggestions, I would perform one last test. Remove the inlet side of the pump (the side with the strainer/filter. Is there water flowing from this line? If yes, reconnect and disconnect the outlet line and turn on the pump. If it just dribbles out you have a bad pump (broken diaphragms). If the pump changes sound and starts to pump water, you had air stuck in the pump that caused cavitation. The pump can't pump air so it gets stuck inside the pump until you let it out. Hope this helps, Fred