There are four possibilities.
The propeller is broken- check that the ends have not snapped off.
The front propeller housing is damaged, on the inside of the propeller housing are 4 stop tabs that stop the prop when it goes backwards, if these are broken the pump will spin backwards.
The prop magnet is not able to move up and down on the shaft due to calcium deposits, clean the shaft and make sure the drive unit can freely move up and down.
The drive unit was reassembled upside down so the disk is on top, make sure the disk is at the bottom.