Rear Shifting Frustration

was just going to say cables need changing - make sure you remove any burs in the ends (before you install the outer end caps) as these can ruin cable runs.

Check how freely the cables are running in the outers. You can do this by hand sometimes.

I've got the same issue on my  steel winter bike! There's one gear where you have to shift two harder and one easier to get the gear. I suspect the gear hanger isn't perfectly aligned.

Funnily enough my winter bike has exactly this problem.

Tiagra 4600, relatively new cables and cassette.  The granny ring and small ring are perfect, so I think the adjuster screws are right.  But ever since I replaced the cables there is one gear in the middle of the cassette that just doesn't quite want to shift up (i.e. to the larger cog).

I've not really played with the adjusters yet, as it seems a little odd that the other 9 gears are perfect.

Have you checked the hanger alignment? If it's a bit out it tends to cause problems at the lower range (Bigger sprockets.)

Are you familar with B screw adjustment?

if it is just one cog, could one or more teeth on that cog  be damaged or out of alignment or something?

If you keep everything else the same and swap out the cassetted that might isolate the problem.

As its a cog in the middle of the block try using the barrel adjuster. Not wanting to shift to an easier gear on the cassette can be because the cable tension is too low. As there are different jumps in teeth numbers between cogs not all shifts are equal. Use the barrel adjuster at the RD, turn it 1/4 of a turn anti clockwise at a time until the problem shift is eliminated, test all the other shifts up and down the cassette to make sure they are working ok (count the number of turns so if this doesn't work you can reset it).

If that doesn't work it could be something to do with the new cable, the inner could be catching on the outer (this can happen if you don't file the ends of the outer after cutting them), or there might be some dirt or something else inside the cable.

Good luck

Check the end stop adjustment on the rear derailleur, there is a high and low screw.  Ideally the derailleur needs to move just beyond perfect alignment as you select it(this will aid chain jumping onto bigger gear) and then align correctly as you release the lever.