Originally Posted by
urbanknight
Assuming you did everything else correctly (including adjusting the cable and limit screws, and using a longer chain for that bigger cog while not exceeding the derailleur's max wrap), I'd place a wager on a bent derailleur hanger.
Yes, its only a 11-28, which Shimano Ultegra 6700 RD should handle. Also longer chain yes.
It does actually look like the hangar might be slightly bent!
Is there any clever way to check spring tension in the rear derailleur? I have cleaned and re-lubed the RD, but I dont know the springs have become weaker.