Good looking game man, but it is not clear that only the boxes move through the portals. You did explain a lot with the little hints, so I guess this one sliped through. In the end I got stuck on level 8. It would also be nice if the portal points in the direction of the button you press, so point left if you press left arrow, point right if press right arrow. Rather than having use the buttons to only change 90 degrees at a time.
There was also a bug that I encountered, in the level were you unlock the portal, if you restart the level after picking up the machine. It is already unlocked, before you pick it up again on your next attempt.