The 2 wheels shown aren't actually loose per say but rather not actually  in contact with the rails. As you can see they are diagonally opposite each other. As I've said previously i only noticed  it when cleaning and regreasing the linear rails, i normally just apply  the grease with my finger But after I'd watched an xtool video on maintenance they applied the grease to the wheels and rolled the gantry back and forth to spread the grease evenly that when i noticed the issue. I have submitted a support ticket. I've been informed that a return label and replacement S1 40w machine are hopefully being organised, however it's the Chinese New Year and Spring festival so it may take afew days to sort out as everyone will be having a holiday.