OK, I was hoping that Robert would chime in but you will have to put up with me.
View attachment 31082
Click on the diagram above & print it out.
The bit that changes the gear is item 24
This moves item 8 which allows the two keys Item 10 to lock into the square slots on the gears
Items 18-19 & 20.
Changing gears while moving can break these keys which will jamb the gears.
The springs ( item 3 ) occasionally break which will also jamb the gears.
Externally you can get debris stuck around the cable cleat ( item 27 ) and if the cable moves but the box does not change gears then the clamp nut & bolt ( 53 & 54) might have vibrated loose.
This is a very robust box ( not as good as the 2 speed it replaced ) and down here is used by a lot of commercial lawn care contractors.