I had the same problem with a 1225 cartridge for my shower. Hard to explain, but before you put the cartridge all the way in, pull the plunger piece (not sure exactly what it's called) out a little bit. This should seat the cartridge in far enough so you can insert the horseshoe pin in correctly. It took me a while to figure this out, trial and error. I am a novice
Thanks wald, can you describe what the you mean by the plunger piece?