Had two do the same recently, one six months old so back to crc
who are returning it to SRAM! The other just over a year so I took
it to bits and had a fiddle, on this one everything seemed to be in the
right place but def not working,noticed that the plate that slides under
the red disc was sticking so removed the black spring guide/retaining
pin by the cable adjuster and regreased then put the pin back in
(used threadlock) and reassembled the shifter and it now seems fine!
I don't no how or why this tightened up but by loosenig a touch seems
to fix it.