I found a similar question here but no practical answers, wondering if anyone has encountered anything like this.
I have a 105 5700 long cage rear derailleur with Roadlink extender installed and an 11-34 cassette.
When downshifting (from smallest to largest sprocket) I have no problems, but when upshifting the derailleur skips a gear twice. In other words, when in the lowest gear, I shift once and nothing happens. Shift one more time and the derailleur skips the previous gear right away. The derailleur itself moves as it should but the chain stays on the previous sprocket.
The derailleur and the hanger are straight, the cables and the housing are new. I have a 38T narrow-wide chainring in the front.
Here is a sketchy diagram that shows which sprockets are skipped:
I couldn’t come up with a reason for why this could be happening except that the difference in the number of teeth in the lowest gears of the cassette is simply too big for the derailleur which worked just fine with a 12-28 cassette before.