Adjust rear derailleur - strange behavior So I'm trying to adjust and fine tune the bicycle, but there's something that quite doesn't match up anything I've seen or read online about rear derailleur adjustment, most manuals go this way.


Adjust limit screws.
Fine tune cable tension.

2.1 if it's not going from the largest gears to the smallest, tighten, remove tension

2.2 if it's not going from the smallest gears to the largest, loosen, add tension.


My problem is that after getting the limit screws right, it fails in both directions, that means, going from the largest to the smallest and from the smallest to the largest both fail, so adding and remove tension won't work as fixing one would only make the other worse. However this only happens in the higest gears, the climbing gears shift perfectly, where the fast gears would randomly fail in both directions.

Basically everything is new, and yes, it is definitely matching parts, it was working well a week ago, now it feels like the changes in the smallest cogs are too small to trigger anything.

I understand how all the mechanism works with the exception of the B-screw.

Just a comment when I set up my limit screws it seems like I can push the derailleur a couple of millimeters in the horizontal axis and an entire centimeter in the vertical axis, only in the smallest cogs, I could swear to god that's why, it's sticking to the previous position and hence it fails in both directions unless cable tension is able to overcome that, except that there's not enough cable tension to overcome the stickiness of the derailleur in the smallest cogs, but the derailleur is new, some sort of setting or what?


                
            

EDIT: I just added oil to the derrauilier and it seems less sticky and it got better, but it still fails a lot to switch.
From the video it does indeed look like the derailleur is being prevented from moving properly. You need to determine where the cause of that is. I think that most likely it's the cable or housing that is the problem.

1) Examine the shifter cable and housing, make sure there are no kinks, tight radii bends etc. Make sure the housing is fully inserted in any bosses, including shift levers and derailleur barrel adjuster. Make sure there are no obstructions to any sections of uncovered cable.

2) Remove rear wheel, shift gears at the levers and check the derailleur is going through its full range of motion and not sticking, especially on upshifts (cable loosening).

3) Disconnect the shifter cable from the derailleur, move the derailleur through it's range manually. Check there is no stickiness and the parallelogram spring is positively returning the derailleur to the high (outer) position.

4) Hold the shift cable at the derailleur end and put a little tension on it, run the shifters through all the gears. Check the cable is moving freely as you go through all the gears.

This should make it obvious if it is the derailleur itself that is sticking, or the cable, or possibly the shifter.