Shimano has identified 100 Causes of Bad Shifting, and they just couldn’t help but make a list for your edification.
The causes run the gamut from the obvious, like frayed or splitting gear cables (#17) or cable clamped between kickstand and frame (#21); to the obscure, like freehub body mounting bolt loose (#42) or chainstay angle exceeds specifications of front derailleur (#79). My favorite is sticky drink spilled on front derailleur (#38). What can I say, they are thorough!
If you are having shifting issues and no amount of diddling, adjusting and noodling has helped, take a look at Shimano’s list for some ideas of what to check out (at the very least, it’s good for a laugh!)
And remember, if you still can’t figure out what’s wrong. . .
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