Running 3×9. The chain goes up and down the cassette just great except for one particular change in both directions. What’s going on?

Cassette borked? Rear mech tired??

Cable tension would be my first guess.

Which particular change?

Mech wear or cable contamination.

new cables fitted this morning, problem happened this evening.

changing from 1-2 (1 being 34t low). sometimes goes up to 2 ok, but sometimes misses 2 after clicking, then goes from 1 to 3 for next shifter click.

I’d get your hanger checked for alignment too, poor shifting at one end of the block is a classic symptom.

I’d second coatesy on this one – every bike i get in for servicing gets a check over with the Mech alignment tool before i do anything to the drivetrain/gears – i’d say at least 25% have bent mech hangers of some description.

Limit screw not set correctly, so the jockey wheel is too far inboard of the 34T cog (even slightly).

Perhaps not explained very well.

Looking from behind bike towards front with rear mech in 34T position, if the jockey wheel sits left of centre line of the 34T cog (even slightly), sometimes a shift to cog 2 won’t actually result in a shift.

Yep, cable trouble, straight thru run or interrupted? Latter is more prone to contamination