I have mangaged to remove the Moen 1248 cartridge from my tub. Now I am having trouble getting it to seat all the way to the bottom. Is there some trick that I am missing?

Did you grease the new cartridge with silicone gel or p[lumbers grease? Greasecthe cartridge and the inside of the valve body, how did it work?

Pull out the stem so that the cartridge is in the open position. In the closed position you are trying to compress the air and any water present as you push the cartridge in.