Sorry, I was thinking of my old GPR on the clean out screw.

see if you can find a nut at a hardware store that the new nipple can screw into. You could force it through that any maybe clean any burr off a thread that might be stopping it. If the old one fits that nice I would suspect the threads on the new nipple.

There may be burrs on the threads of the new nipple. Try folding a piece of fine sandpaper so it's kind of like a knife blade and polish the threads.