On both my old Bianchi Volpe (27 gears) and my new Felt (double crank with 18 gears), I have had the same problem with getting the right adjustment on my gears.
The problem occurs when I downshift (move to larger rear cog) from 7th gear to 6th. Even after an adjustment at the LBS, it will soon not want to downshift - either it will delay too long or not downshift to 6th at all unless I nudge the shifter over a little extra before releasing. It upshifts (to smaller cog) just fine.
I asked the LBS about this when I had them replace the chain four days ago (no, it isn't the chain being old or stretched - it happens regardless).
As they were checking the gears after replacing the chain, the guy said that if it gets that way and even adjusting the screw thing in back, which I had tried over and over with no improvement (turning a very little bit each time), then he said the hanger might be bent. But he said mine looked fine. Still, he got out a tool to check it and maybe made a very minor adjustment.
So, I'm riding home and found I had the same problem going to 6th (but maybe not quite as bad) AND it had a little more of a delay sometimes downshifting to 8th and 7th as well.
I turned the adjustment a tiny bit and everything seemed fine the rest of the way - even going down to 6th.
Two days later I rode 54 miles. For the most part it was fine except the problem going down to 6th was back. I had to nudge it extra to get it down.
I didn't mess with anything - just figured next chance I got I'd go to the shop again to pursue the matter.
Then today I rode 43 miles. I decided to try adjusting it a tiny bit more to see if the problem went away some miles from home. I can't recall when I did that, but it wasn't more than 8 miles.
Well, sometimes I felt the shifting wasn't quite right. Eventually I found it wouldn't UPSHIFT from 2nd to 3rd so, okay, I simply adjusted it back where it was so - theoretically - only the 6th gear problem should remain.
But then things got really weird. In the lowest gear, it didn't sound right with the chain. Then the chain kept jumping between gears (typically between 1st and 2nd though I think it may have extended further than that but since I was climbing my last hill, I didn't get to check much).
When I got home, I set it on a little stand so I could turn the cranks and experiment with the shifting. Nothing seems to work no matter how I make that adjustment.
Now, I have two questions. The main one concerns the repeated problem, with two bikes, at downshifting to 6th gear from 7th. What are some things that could cause this?
The other has to do with this present disaster where things are really screwed up! I just can't imagine how that small change - which just put things back the way it should have been before - caused such havoc!