I finally got around to fixing 1 of my sticks, as eikenb said the switch had moved.
Basically there's nothing to stop the top switch moving to the right a few mm or so (2nd photo), that gap (see 1st photo) is a bizarre design decision and makes me wonder if they had intended to glue the switch in place? Maybe the right hand 1/2 of the grip was used with a different version of the stick which had a larger switch??
Anyway I put a few layers of insulation tape around the edge of the right hand side of the inside of the grip (3rd photo) to (hopefully) stop the switch creeping to the right too far, but I'm not entirely sure that the tape will be hard enough for that though. If you try this repair, make sure you don't put too many layers on or it'll will stop the back edge of the button moving down enough to work the switch, and don't tape over the slots, or you won't get the cover/grip top back on.
If the tape doesn't work in the long run, I'll ask a friend of mine who has a 3D printer to see if he can make a little plastic sleeve to go over that edge.
This is where the switch should be.

This is where the switch ends up moving to (for whatever reason, for me it was simply using it), only when it gets right over to the other 1/2 of the grip does the button stop working.

This photo shows the tape I've put in place (which won't stop it moving completely, but hopefully it'll be enough. I didn't tape put around the edge of the switch as it didn't look like there was enough surface for the tape to grip on properly), with the switch back to where it should be.
