Originally Posted by
RoadTire
If I understand correctly, the problem is going from the larger cogs to the smaller cogs of your cassette? So as you release cable tension by shifting, allowing the RD to return to low-normal, it tends to not move. That indicates binding cables as a first, and easiest guess.
Increased cable friction would also be my guess, but without knowing how old the cables are, it would only be a guess. Ultegra cables have a low friction coating that could wear off in time increasing cable friction. Since this is the third year that Ultegra 6800 components have been on the market, the cables could be up to three years old and it may be time to change them