Hey

I have skill 100 in smithing and every perks including Dragon armor. But, when I try to improve my glass bow (legendary) I got the message "You lack the required skill to improve this item".

Other item I can`t improve: All the Thieves guild armor above (Flawless), all the enchanted weapons above (Legendary)

Is it something I am missing?

Try use a blacksmith potion. There are hundreds of threads you could have used to find that out. also legendary is as high as items go