Le Mechanic

The easiest way to diagnose the issue is to isolate each component individually. The most common issues with the symptom you've described is most likely 1 of 3 problems.

First thing, double check and make sure your small cog is correctly clocked into the splines on the free hub body. If it's not engaged into the correct splines, the cog will sit out a little far making it difficult to drop down to the small cog with everything else shifting fine.


The second thing to check is if the derailleur, with the cable unhooked can manually drop into the small cog. With the bike in the repair stand, rotate the cranks and manually push the derailleur up the cassette into the larger cogs. When you let go of the derailleur and continue to rotate the cranks, does the derailleur return spring easily pull the derailleur down into the small cog? If not, I suspect the high/low limit screws need to be reset. If the derailleur was adjusted to work on a damaged hanger, the limit screws will be off when paired with a new straight hanger.


If all of the above is good, it's probably a cable friction issue. Cables cut straight? No kinks in the inner cable? Under BB cable guide clean with cable routed correctly? You can check if there's any friction by pulling the disconnected end of the inner cable while shifting through the indexes on the shifter. It should have no/very minimal resistance.


It could be an issue with the shifter, but that's pretty rare.