How many points have you got in Repair skill? You get a certain % for each point so if the item isn't "broken" enough to be beneath your current skill level's % cap trying to repair it will do nothing. Also it rounds % limits down (yeah, of course it does, lol) so if an item is say 3/7 if you got 50% repair ability it won't do jack. I would pick an item that is really broken (like zero or 1 condition left) or quality item with a lot of condition points to practice on so you know for sure you should be able to repair it some. It will work but it is a bit fernickety. I had a trouble getting it to work to start with too.
