Argh! This is triggering some repressed rage for me!
I have beaten Dark Souls 3 times. I have done everything. I have every achievement... except one.
And it's because the achievement just... didn't pop.
It's the one for "get every weapon".
I printed off a list. I went through the game, crossing them off as I got them. I got to the end of the list, got that last weapon and... nothing. Nothing at all. I went over the list a dozen times. I checked other lists online. I checked my inventory over and over and over.
I have all the weapons - upgraded as far as they'll go - and the achievement didn't pop.
So there it is, my shameful, incomplete Dark Souls gamerscore. I just don't have the stomach to replay the game THREE MORE TIMES to try and get all the weapons (since you need a minimum of three playthroughs to grab them all).
I still don't know what I did, or if there's anything I can do to fix it. But that one achievement haunts me because it's the one time in Dark Souls that feels overtly cruel, and I know it's just gotta be some glitch. But considering the game, and having all the others that I worked so hard for, not having that last one is painful.