2007 Trek 4300, Alivio rear derailleur with about 2000-3000 miles on it.
The shifting has been gradually getting worse over the past year. The hanger is straight. I've cleaned and lubed the cable, and have removed as much grit from the derailleur itself as I can.
Here's the issue. Start in the small cog and shift through the gears to large cog to check indexing. Indexing is good. Now from the large cog, release the cable through the gears toward the small cog. The derailleur won't release enough to get to the next smaller cog reliably. If I gently pull on the derailleur body, I can nudge it toward the smaller cog and it will stay well enough to shift, but it will stick again on the next shift.
If I use the barrel adjuster to align the indexing properly for releasing cable tension, downshifting won't make it to the next larger cog. I'm wondering if it's possible the spring has worn out. It doesn't seem like I should be able to pull the derailleur outward and have it stick. By contrast, my 105 RD on my road bike releases all the way, and when I tug outward on it (gently) there is no movement.