New answers tagged shifting
Take a close look at the limit screw - is the derailleur actually hitting it? If not, then it's possible that the spring which pulls it to the right is worn out. If that's the case, you might be able to replace the spring, but unless it's a really expensive derailleur, you're better off just replacing the whole thing.
Just as Batman said, check the derailleur is not bent. If it isn't, but you still can't get the smallest cog I'd be looking to check if the mech hanger is bent. But if you're bike shop is checking the alignment of the mech, they should be able to spot if the mech hanger is bent or twisted at the same time. Hope you get it sorted!
The first thing to do with any rear derailleur problem is to check if its bent. If it is bent, go to your LBS and see if it can be unbent (there is a special tool for checking unbent-ness, Park Tool DAG-2 or similar) -- bent derailleurs cannot be adjusted properly in most cases (and even a minor spill can cause a bent derailleur). If its not bent, then try ...
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