[Closed] Why won't my (road) gears work properly?

Bent hanger?
Cable clamped incorrectly in mech?

Bent mech hanger?? From what you described that's the likely culprit.
Or check the mech, jockey wheels have play?or the mech pivot bolt seized? Couple of things to check out.

Thanks folks, will check those in the morning 👍

As above and check you are in the dropouts correctly as well

Assuming you haven’t done anything to knock the hanger out since it was working with the old bits...Id be having a close look at the cables, the seating of the outer end casings, and the routing into the mech bolt.

While checking the cables make sure the outer reinforcing hasn't punched through any plastic ferrules in the system.

You've implied it's not a new mech...perhaps it's worn?

Suggestions above are sensible. I'd add checking cables aren't too long or short but that's unlikely to be a sole cause.

B tension and chain length worth checking too

Yeah, it's not a new mech. In fact it's the original mech so has around 10/12k miles on it, perhaps time for a new one.

Have you shortened the new chain to same number of links as the old one ?

Another though occurred. My Tiagra gears don't index correctly if the rear quick-release is not tight enough, there is sufficient movement in the gear hanger that it needs to be just so.

I'd guess one of gear cable outer end stops isn't properly seated and there's play in the system.

Make sure the screw adjuster isn't wound all the way out. It will pull straight in lower gears but moves to the side in higher gears altering the ratio.

Loose rear mech bolt or loose mech hanger (if it's a replaceable type).

Tip:- replace the jockey wheels when you change the cassette and chain, they cost buttons.