Originally Posted by FBinNY

Try this.

Shift to the outer ring, then back to the middle. Then shift to low where you have the issue.

Now, shift back to the outer ring by pulling the bare wire away from from the down tube as if you were drawing a bow. Continue pedaling and ease the wire back to shift to the middle ring, and let go suddenly.

If it now shifts, your problem is excess friction in the system, probably in the housing between the bars and frame.