Once had an 833 that would get stuck in 2nd, and moving the side cover a wee bit - can't move it much due to shouldered bolts - fixed that. Not much room for the forks & combs to work, and they can hang up on each other if the cover location is off just a little bit.
I'm assuming here that shifter's properly adjusted, no rod interference, and no damage to shifter, levers, or rods.