For the last couple of rides I have had a problem downshifting into the middle cog on my 10-speed Ultegra cassette. I run a standard 12-25 cassette and keep it well lubed. It's only the middle cog and only when downshifting a single gear. If I do a 2-gear jump it works, but always hangs on a single. I've got the RD dialed in as but can't figure this out.