Originally Posted by FBinNY

Before you take anything apart, try shifting by pulling the wire away from the downtube like a bow string. This simple diagnostic can reveal a wealth of insight.

If the wire comes away freely without anything happening in back, you have a broken cable.

If you get it to shift fairly normally this way then it's a problem either in the lever or housing forward of where your pulling.

If it shifts but is sticky or takes too much force then there's friction in the RD or rear housing loop (rear loop is probably the single most common cause of shifter wire friction)

If it does shift buy needs excessive cable movement, then either a housing is breaking down and compressing, or the cable slipped forward at the Rd pinch bolt.

There are other possibilities, but no other diagnostic tool yields as much info compared to the near zero effort involved.