Out of the blue it’s just decided it doesn’t want to know!
The right arrow key is working, and you can see that it turns the snap to axis on and off (red dashed line), but the movement of the object will no snap to the axis.
Absolutely nothing has changed, I’ve been in SketchUp all afternoon. No software updates or anything.
Tried restarting SketchUp, tried restarting my Mac.
Nada.
Just tried another file and it’s fine, so it’s something in the file that is causing it.
Although how I turned gluing on for that object, I have no idea as it’s not a command that I use (for better or worse). I guess I have inadvertently hit whatever the keyboard shortcut is.
Strange though that I had been moving it around and duplicating it all afternoon without either needing to snap the red axis or noticing it not snapping.
Nevermind though, problem solved and something else learned!