The first thing I do to diagnose this kind of issue is disconnect the cable and shift it manually by pushing on the lower body. In fact, I never have to do this test after, because I do it as part of the initial set up.

Anyway, shift to the 2nd smallest hold it there a moment then let go and see if it shifts to the 11 crisply. Odds are it does, but while there, feel how light the spring load is at that end of the travel range.

Which leads to your likely problem ---- cable friction, probably in the rear loop, and often caused by a loop cut wither too long or short so the ends leave the fittings at an angle.