If you have a Dura Ace Di2 electronic shifter and you find that it cannot shift the rear cassette to a larger gear (downshift), you might have forgotten to put your rear wheel back on in the same gear in which it was taken off. The Dura Ace Di2 rear cassette electronic shifter does not appear to sense its position. My guess is that it has absolute locations for shift positions, and you can confuse it by putting the derailleur back on at a different position.
But wait! If you return your wheel to a different gear it should automatically find the gear it was previously on, simply because the derailleur is in that position. For whatever reason, that may not always be the case with the Dura Ace Di2 electronic shifting system. I have had it happen both ways. Sometimes the chain moves to the right place on the rear cassette, sometimes it does not.
In my case, I solved this by removing the rear wheel and shifting all the way to the outermost gear. When I put the wheel back on, I made sure that the chain was on the outermost gear.
