User blog comment:Bedgehog/HELP! upgrading nightingale armor/@comment-207.28.99.29-20120127163544

You said you have your smithing skills maxed out, but do you have all the perks? That perk in the center of the Smithing perk tree, Magic Smithing, or something like that, is the perk you'll need to take to be able to improve the Nightingale Armor, since it counts as magic armor. If you've already taken this perk, then you're probably bugged and there's not a thing you can do about it unless you're on the PC and know the console code to get improved Nightingale Armor.