On the sixth stage, the boxes have to be placed on the right.
Step by step solution
Go DOWN, push RIGHT and RIGHT. Go LEFT, UP, LEFT, UP, UP, RIGHT and push DOWN.
Push RIGHT. Push DOWN, go RIGHT and push DOWN.
Go UP, LEFT, UP, LEFT, LEFT, DOWN, push RIGHT and RIGHT.
Go LEFT, DOWN and push RIGHT.
Congratulations. You have completed the sixth level. Get ready for the seventh.