i am having issues with 4 out of the 13 weapon attachments unlocking in single player? does anyone know how to fix this? i've used every attachment...

I'm having the same problem, except I'm stuck at 10 of 13, though I've deliberately and painstakingly picked up every weapon and used every attachment multiple times, I cannot get the silencer for the Mk 20 pistol, Hammer, and Feline to stay unlocked.

I've seen several people here and on other sites post about this problem so I'm assuming this is a known bug. Hopefully it gets fixed because I really like using silenced pistols for the campaign, and not having them (unless I find and pick up another one) is dampening the replayability for me.