If you adjust it to shift up to a specific larger cog and have problems going down to that from the next larger cog it's a cable problem like frayed strands, worn housing, housing not seated at the shifter, or bad cable routing because the problem manifests without the position changes that would go with a bent hanger or cage.

This is a bigger problem than it used to be because
- More cogs means less pull for each so a little hangup has a larger impact.

- Under-tape cable routing requires tight bends that increase friction so it takes less to make a problem happen.

- Manufacturers compensated for this with plastic linings that reduce friction when new, but quickly wear and stick (I didn't get 6000 miles out of my first 10 speed right shifter housing).

If the problem varies with position you may have a bent hanger or cage which causes the top pulley alignment to vary based on what gear (front or rear) is selected, although a cable problem is still likely (the derailleur return spring applies less tension as you move towards the small cog).

If you undo the rear derailleur cable clamp then push and pull the cable you can feel it not moving freely when it's bad enough.

Problems were prevalent enough that Campagnolo came up with an official test for cable friction on their latest 10 and 11 cog levers - with no chain installed before trimming the rear derailleur cable to length you temporarily install it protruding from the shifter, hang a 1kg (2.2 pound) weight on it, and make sure the derailleur returns all the way after being displaced from the small cog.

#t=0m45s