I suspect that your problem may be related to housing friction. Shift to the second sprocket then back to high gear, where it's hanging up. Firmly pluck the bare gear wire away from the downtube like a guitar string. If that's enough to complete the shift, it confirms my guess.

You might also consider trying to shift from high to the next sprocket and back by pulling the wire from the frame, rather than the lever. If that works even if you release the tension slowly, the friction problem is in the handlebar to frame section of housing, if it only shifts with a quick snapping of the wire, then the problem is likely in the chainstay to RD loop.