cartridge replacement probs for shower

[gnowlin]

just got my first AS. 2004 Intl CCD. on the first trip out I noticed leakage in shower, water would not shut off . so i watched the cart. replc.video that's posted and decided to replace the old cartridge. got a #1222 from Home Depot ( not a Moen but it says it is the replc for a Moen) encountered a prob in getting the retainer pin out; it is behind the shower wall, so i disassembled everything and managed to remove it, very difficult as it is obstructed by the white plate that guides the screws in from the outer chrome handle plate. did get it out (? on how its going to get back in). the new cartridge will not fully seat, tried to get the old one to seat and it will not either. i get the cartridge in about 3/4 of the way and that's when it stops. i've put lube on the cart. and felt for any obstructions but cannot find anything. any one else had this prob? looks very simple in the video but i'm fustrated that mine is not that easy. not sure how to get the retainer pin back in either HELP