I've got an issue that I've been trying to fix for more than a week. I broke my derailleur+hanger because I changed my cassette from a 11-28 to a 11-32 with a short cage Dura Ace 10s 7900. I replaced the hanger and bought a used Ultegra 10s 6700 (good condition and completely cleaned it).
I installed everything correctly, limit screws + indexing works fine when bike is upside down or when I change gears while the bike is in the air. But on the road the last 3 gears (11-13-15) don't work smoothly, I can't switch to a smaller cog perfectly, there's always a delay and then it snaps really hard, even while the derailleur cage and jockey wheels are nice and aligned with the cog meaning indexing is good. Can't be the hanger either, it's new and I tried checking if it was straight by screwing a wheel into the threading and the wheel was completely parallel to the other on the bike.
I also broke a link in the chain so I had to make it smaller and it was too short (couldn't be in the big ring + big cog). I now changed the chain to a new one
I went back to the 11-28 after breaking the derailleur and its hanger, so I thought maybe the old cassette is used ? So I put a new 11-30 which is ok with the Ultegra 6700 mid cage I have now
Today I decided to look at the housing and cable, and indeed the cable was completely messed up at its head (it was all mushed inside the shifter) so I thought great that was the issue the cable was torn apart and was causing too much friction, but now with a brand new cable and housing where I put wd40 to lube it a little bit, it's better but still not quite good for the smallest cogs.
So now I have : new chain, new cassette, new cable, new cable housing, new derailleur, and it's still not working smoothly. The shifter works fine as well I tried it when I changed the cable and also put wd40 inside to clean it.
I also bought the same groupset (2 shifters, derailleurs and brakes) and put the rear derailleur and it was a bit better but still not good (maybe the derailleur spring is used ? I cleaned it and it snaps back really good when I push it with my hand...)
My theory for now is that there's still a little too much friction in the housing or somewhere and the derailleur spring can't pull back enough to a smaller gear.
I guess next step is changing the shifter, I tried looking at the one I have on the bike rn when I replaced the cable and housing and it clicked really nice, I cleaned it with wd40 also.