I'm level 100 smithing on Skyrim with all the perks bar the heavy armors ones. before patch 1.7 I could improve all light armor to legendary but now I can only get it to flawless? anyone else having this issue? I just joined the dark brotherhood and i'm trying to improve the armor given to me to legendary, also trying to do the same with the nightingale armor but still to no avail...only flawless...thanks guys

Those specific armors require a skill greater than 100. Drink a potion or wear stuff enchanted with Smithing.

If an item isn't associated with any Smithing perk, the highest level you can upgrade it to is flawless. It's always been this way.

You'll need potions or equipment fortifying smithing to improve them more.

Some armors, such as those you mentioned, you need a Smithing level higher than 100. I think it is 168, but not sure. I just know i wore three items that raised my Smithing 75 points. Then again, I am not sure if these two specific ones can be improved past Flawless. I will have to check my set when I get home to see what level it is at.

However, those Armors at Flawless are probably putting you at the armor cap already. Especially if you have the Matched Set Perks that give bonuses for All Light Armor and Matching Set. Check to see what your DR is now, you may not have to improve it.

I think the OP's point got missed. He was previously able to improve an item to a higher tier without using additional stat boosters. I certainly don't remember reading that smithing difficulty was being changed in any patch notes so I wonder if his save.game is bugged or if the smithing perk tree got bugged with the last patch.

The way to test for bugs then is with a proper "control": Take a newly forged piece of one of the same armors that could be made Legendary before now, and see if it can become Legendary (or not) now. (If you are only trying on DB or Nightingale, that is not a proper control for testing the issue. As other posters have said, they add an extra variable.)