Start with making sure the shifter linkage moves freely. Take it apart and clean it, then lubricate the pivots. Inspect for bent parts, or binding. Shifters most often 'stick' between upshifts, because the shifter relies on gravity to return it to the center of the throw between shifts. When you make a down shift, the spring pulls the shifter back 'up' to center, allowing the next shift.
Things get bent in a crash, or when someone doesn't know to maintain the shift linkage and start stomping on the shifter.