Originally Posted by Drew Eckhardt

1. The rear loop can go high-friction before the rest of the housing - it picks up the most dirt and can get a sharp bend where it meets the derailleur.

With your bike on a repair stand you can shift to the smallest cog and then work the derailleur by pulling on the cable to see whether your issues are with that part of the cable or elsewhere.

If that's the case a foot of housing (left over from the last replacement) will fix the issue.

2. Strands at the shifter head break and hang up on the housing. When that happens a new cable is all it takes.