SKB is correct, check your tabg screws. You might need to tighten or loosen them, compare each one and count the number of turns during installation to the old stock since it works fine with that wood. There isnt much wood in the head of a Model 21 stock to begin with and he Miller design usually requires more to be removed. A little inletting black on the top of sear legs and selector components will help eliminate the possibility of wood interference being the issue.