You need to figure out what is stopping it. Degrease it. Use cold blue, dye-chem, or sharpie marker to coat the likely places it might be binding. Screw it in. Work it back and forth at the pont it binds. Unscrew it. Observe the bright spots. File off the bright spots. Even better use a lathe if you can. Repeat the process. You want it to bottom out on the barrel. Well the barrel and the inner shoulder at the same time. But I sounds like that will not happen. Can't win them all. Good luck