Have a look at the cable where it bolts to the derailleur. It may not have been put in properly. I did this accidently/stupidly once on a dura ace and it wasn't immediately obvious - didn't have it running through properly at the derailleur. All the gears were working spot on except on the 11. The guru mechanic at my dads mates shop (former national team mechanic) picked it up and it wasn't even immediately obvious to him.