When you try and fix a item in a anvil I doesn't do it??? so you go to hub and come back and you get your xp back and your diamond but it does it in a endless cycle
I just tried fixing my armor and it works. If your armor has name (Like those in kits) then you would need to change the name of it before you can fix it.
Zrinne, it's not just for named items. Nametags, and general items *usually* won't repair. So far I've only been able to successfully repair an item once.