I thought upgrading weapons required smithing, not enchanting?
I meant Smithing. My bad!.
To get enchanted items above flawless ie nightingale bow + daedric artifacts, you'll need smithing at 100 with arcane blacksmithing and daedric smithing perks to play it safe, having either alchemy or enchanting at 100 as well helps.(use crafted enhance smithing potions or 4 pieces of peerless smithing gear).Note that peerless smithing equipment can be either crafted or found at stores or in random loot depending on your level.
I'm gonna try that. I had another character that had every perk on Smithing and it still wouldn't let me upgrade the bow and other items beyond "Flawless"