I'm sorry that's happening to you. As long as you're typing "Deckard Cain", with capital lettering, there's no reason it should be wrong. I can give the code a look and reupload whatever version you're having issues with, but there really should not be any problems... The only one I can think is if you're using a translated version or something and that messes with it.
The translation patches are unofficial.
P. S. I'm doing a massive rewrite of the entire game so far and currently it's in beta available only to Patrons, because I'm not gonna release a half-finished mess to the public. In that one, you'll be able to unlock secrets from replays, so you'll never need to go back if you miss something