Flow-Rider
Burner
It might be worth replacing the cables, sometimes they slightly bind enough to resist the spring on the derailleur.Brains Trust.
Never had to deal with this before because every bike has just worked.
Sunrace 12sp cassette, XT chain, XTR derailleur.
Goes into larger cogs no wukkas, refuses point blank to come off 1st gear, but then for the rest of the cassette the cable is too loose and will double shift.
I've had a bit of a play with the B-screw, tweaked the cable tension. That's about it.