Installed a 2011 Dura-Ace 10 speed chain ) to replace my stretched-out SRAM PC 1090-r, which was a very good chain, and now I get chain skip only when I'm in my 21t cog (Dura-Ace 12-13-14-15-16-17-19-21-24-27), which is a cog I rarely use, so it's not worn.

Very annoying. What could be the culprit? The Sram was the same length and did not skip.