 You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
I've been fettling. Fitted a smaller chainring and removed a couple of links from the chain. No hiccups.
Then I take the bike for a spin. Shifting up the cassette from 10 to 42 is faultless. Shifting down the block is weird. From 42 down to the second cog is fine. Moving the next cog requires two clicks of the shifter. Next one moves down quickly enough, but is a little rattly. One more click, slick but a little noisy. Down the rest of the block, everything seems ok.
Dérailleur hanger looks straight. Can't think that it's cable stretch as that would make the mech reluctant to move up the cassette, not down, surely?
Anyone else had this issue with X01 or other SRAM 1x systems? Help!
Just sounds like you need to adjust the cable tension to me
Check the b tension screw. Sounds like it's too far out.
I've got this on mine currently. I went to a smaller chainring ages ago but it's only just started doing this and it's on the third cog down too, shifting up the block is fine, but going back down the block I have to go to cog four and then back up to three if needed.
I was wondering if it's time for a new cassette but it looks ok and also measured the chain with the park tool thing and that's also fine. I'll need to get fettling.
Please post up if you find a solution.
Two things to check:
1 - chain length on SRAM 11 spd is important. It runs longer than you might expect. in the 10T the jockey wheels should almost be horizontally in line, with the lower only slightly... lower. Looks weird but is the right way of doing things.
2 - you need to set the 10T limit screw quite wide, wide enough so that the jockey wheel would be just on the outside of the 10T cog if you could move it up that far to the cassette. Then set the cable tension in that position. Sounds nuts but it does work, and the extra tension this provides will help pull the chain back down through the block.
You may have both these right of course!
Thanks for suggestions so far. I've done less than 100 miles on this groupset so the cassette is pretty much new, as is the chain. And I'm pretty sure that chain length is fine. I'll check the pulley position relative to the 10 sprocket.
Check your drop out and Mech are straight - ive seen them come bent from the factory
Cheers
Zoo 
This looks handy:
Just sounds like you need to adjust the cable tension to me
This, wind it right off then start again from the smallest to the largest...
In case you were wondering, it [i]was[/i] the B-screw. Needed tightening by about 1.5 turns. That's quite a lot...