I can't upgrade my Thieves Guild armor, boots, gloves, or hood past 'flawless'. I've unlocked every perk in smithing and it still says I lack the required skill to improve any of them. On UESP, it says one bug won't let you upgrade enchanted armor if you already made smithing legendary. I thought this might be the cause because I have made smithing legendary, but I was able to improve all my gear to 'flawless'. I've tried getting other Thieves Guild armor and it does the same thing, it only improves up to 'flawless'. I'm able to improve other armor that is enchanted to 'legendary'. I haven't been able to find any known bugs specifically about the Thieves Guild armor set, either. Any suggestions on what to do? I really like this set of armor because I like playing as a thief, it just svcks if I can't make it legendary. If anything I'll just have to tough it out and dodge enemies more often (if I get caught)

Thieves Guild armor isn't associated with any perks, so it can't be upgraded beyond flawless, unless you get your smithing skill past 100.

Make a full set of Smithing gear, Clothes, Gauntlets, Amulet, and Ring to increase your Smithing skill.

You can also do the Alchemy/Enchanting/Smithing exploit.

What is the alchemy/enchanting/smithing exploit?

http://elderscrolls.wikia.com/wiki/Forum:Skyrim:Alchemy/Enchanting_Loop