When I rightclick twice during game and move my mouse, a select box appears and follows my mouse (as if I was holding down leftclick) but when I select some units and left click to select them (using the double rightclick box I just created) nothing happens. So I repeated the double rightclick again, made the box surround my units and rightclicked. Instead of selecting them, it assumed that I already had selected them and made them move. Try it.
