Hold the shaft between your finger and thumb. Spin the magnet. It should spin smooth with no restriction. If it spins freely then Clean the hole in the MJ real good. Plug in the mj and hold the rubber boot on the prop end of the magnet and slowly slide it into the mj hole. If the prop doesn't spin then the unit is shot. Sometimes you get grit between the magnet and the shaft preventing it from spinning(potentially causing the pump to overheat). Other times you'll get a scale build up in the hole. The other problem is if the shaft is not centered and the magnet is sticking to one side. Hope this helps.