Having a bit of grief getting my rear mech to index properly, it’s an XTR clutch mech on a 3×10 setup on a new-ish bike.
Have followed my usual favourite instructions, from the Park Tools website, and it seems to be set up just fine when moving up the block, i.e. from the smallest sprocket to the largest.
Shifts quickly and quietly and doesn’t miss any gears.

However, shifting in the other direction, from largest to smallest, the same is not the case. It will miss the odd shifter input and then either shift 2 on the next change or randomly catch up on that missing shift all by itself. And be a bit rattly in between.

Any ideas ?

Much appreciated

Inner cable sticking or snagging somewhere would be my first port of call, more so if it’s not got a continuous outer.

As above, also worth checking you’ve got the B-tension correct, too close and the chain will be trying to go across the sprocket’s teeth rather than up and over them.

does it do the opposite if you slacken the cable tension,
i.e. from biggest to smallest it indexes fine but then from smallest to biggest it doesn’t index correctly?
this would then show if the cable is slick enough and then maybe something else.

is the hanger straight? is the mech bent? any play in the mech pivots?

Sticky cable or bent hanger I’d say