Yeah, I had this problem with the X9 mech on the new bike I picked up last week. Wouldn't shift onto the smallest cog unless I happened to hit a bump at the right time.
In my case I think it was a combination of a relatively weak spring and stiff joints. Shift into top gear, and if you can then push the mech into line with the smallest cog by hand, you've got the same issue.
Googling turned up a few people with similar problems who sent the part back to SRAM and got it replaced under warranty. I didn't fancy faffing about with the shop unnecessarily so I tried seeing what I could do at home first.
I took the mech off the hanger, and pulled the cables out of the guides without disconnecting anything. I then soused the thing in plenty of WD40 and spent a good fifteen minutes flexing the parallelogram bit of the mech backwards and forwards by hand, concentrating on the bottom of the range, where the spring is at its shortest extension. This seems to have loosened things off enough and I've not had the same problem since.
(Hello all, btw, new here.)
