Usually this would affect the whole of the central locking and wouldn't work period; but is the boot, or one of the doors open slightly? It's shut, but not fully? This was a common problem for new Honda drivers who couldn't lock their car thinking the central locking was f**ked.
And I'm sure you've tried'; but try turning the setting back to normal, then back to auto lock.
Another thing to try, make sure your left mirror is set to go down when reversing. When you first start the car, put it in reverse, let the left mirror go down. Once you drive off, the mirror should revert at the exact time the central locking should activate. By doing this, you can see if it's just the central locking, or maybe an issue with a sensor going over the required speed to make these bits happen. Just a thought at least.