There are 3 possible causes and we would have to narrow it down to which cause it is first.
1) The front propeller housing may be on crooked, please check that it is secure.
2) The propeller may be defective or loose, try swapping prop assemblies between pumps and see if it follows the propeller.
3) The disk may be the older 6025.740, since last October 6055 has included the 6055.740 disk and you can see this at the base of the shaft, it will have a dark blue ring around and under the black plastic disk, you will need some sort of small light to look down into the well at the base of the shaft and see this part.