Originally Posted by
joejack951
Your LBS is apparently incompetent. The problem could be the derailler itself, a cable tension adjustment, a limit screw adjustment, or the derailler hanger. Most likely the latter as it's often overlooked.
Correct. Dura Ace rear derailleurs routinely handle 11T cogs and are rated for a 27T largest cog and often work with larger ones. There is nothin inherent wrong with the Dura Ace so something else needs to be adjusted or aligned properly.