Remove the shifter and clean the exposed shaft very well. Pull the side cover and clutch, and you can remove the shifter shaft assy from that side by pulling it through the cases. Note the assy of the parts for reassembly. The fork is welded on the arm, both it and the sliding "scorpion" are stamped steel parts. There is possibly a slight distortion(from the stamping or welding), maybe some weld spatter, and sharp edges. Clean up the latter two with a fine file and some scotchbrite. Check straightness of the sliding surfaces and and clamp the parts in a vise or strike with a hammer between two pieces of flat stock to correct if neccessary. The assy has to slide very smooth. I've done this on all three of my bikes as PM during initial setup. It was common a few years back but from what I can tell is unheard of the last couple years, so I guess QC on this assy has improved. Not a big deal anyway. The other thing is that the assy sits low in case and any debri that settles there could foul it, especially if its borderline sticky to start with.
Somewhere in my many USB drives I have pics of this I took when I replaced my starter gears two years ago. I'll try and find them but for now refer to the manual.