Possibly the following link will help you.
Sounds like somebody in March had the same kind of problems I am having with replacing my shower cartridge. I have a new cartridge part #900-022 and it came with gasket and screws, but it also came with what appears to be a small plastic impeller. I'm not sure what this does or where it goes. Anybody can help?
good luck, you are gonna need it.
Just put the little white piece in the same way it was in the old one. Doesn't work right without it. Check the operation out carefully after you turn the water on. The out-of-box failure rate of these cartridges is very high.