I have removed the old cartridge and put the new one in. The problem I am now having is the locking ring will not go back in and I think it shouldn't be that tough to slip it back. What am I doing wrong?

Hello Ramfyr, check your installation. It is possible that the cut-outs on the end of the stem are not aligned with the slots in the body. Also, make sure you pushed new stem ALL THE WAY in. Come back and let me know how you did. Milo

You do know Moen warranties their products for life don't you? They'll send you a new cartridge for free. If you need instructions on replacing a cartridge click on;
Installation Help / Animated Tutorial - Installing the Moen 1225 / 1200 Cartridge (http://www.moen.com/consumersupport/installation/moen0133/moen0133.cfm)
Good luck, Tom

You some times have trouble seating the cartridge because you are having to compress the air trapped inside when you inserted the cartridge. Open the stem to let the air out.