Can anyone help me, figure out what is wrong with this function I have, I am trying to get the objects to bounce back as soon as they hit the top  and bottom of the Window, but when I run this  one of the objects stay on top and  the other keeps on going.

(define (up-and-down y-pos-pair)

  ;;get the balls 
  (define b1 (first y-pos-pair))
  (define b2  (second y-pos-pair))
  ;; move the balls (struct ball (color ypos velocity trip) #:transparent)
  (set! b1 (ball (ball-color b1) (+(ball-ypos b1) ball1V) (ball-velocity b1) (ball-trip b1)))
  (set! b2 (ball (ball-color b2) (+(ball-ypos b2) ball2V) (ball-velocity b2) (ball-trip b2)))

  ;;bounce the ball on the borders
  (cond
    [(>= (ball-ypos b2) HIGHEST-Y-POS)(set! b2 (ball (ball-color b2) (- HIGHEST-Y-POS 1) (* ball1V -1) (ball-trip b2)))]

    )  
  (list b1 b2)
 #|(let* (ss
         [b1  (ball 'red (+ (ball-ypos(first y-pos-pair)) ball1V) ball1V (ball-trip (first y-pos-pair))) ]
         [b2 (ball 'blue (+ (ball-ypos(second y-pos-pair)) ball2V) ball2V (ball-trip (second y-pos-pair)))]
         )
    ;;(list b1 b2)
    (cond ;;
      [(>= (ball-ypos b1) HIGHEST-Y-POS)(set! b1 (ball 'red (+ (ball-ypos b1) ball1V) -1 (ball-trip b1)))]    
     )
   (list b1 b2)|#    
    #| (cond
      [(>= (ball-ypos b1) HIGHEST-Y-POS)()]    
     )
     (cond
      [(>= (ball-ypos b1) HIGHEST-Y-POS)()]    
     )
     (cond
      [(>= (ball-ypos b1) HIGHEST-Y-POS)()]    
     )|#
    )
  ;;(list (ball 'red (+ (ball-ypos(first y-pos-pair)) ball1V) ball1V (ball-trip (first y-pos-pair)))
  ;;(ball 'blue (+ (ball-ypos(second y-pos-pair)) ball2V) ball2V (ball-trip (second y-pos-pair)))))
  ;;(list ball1 ball2)