[plt-scheme] match question

Could anyone help explain why the following program doesn't work? Many 
thanks.

Chongkai


#lang scheme

(define (make-nil)
  '())
(define nil? null?)

(define make-::
  (match-lambda
    ((hash-table ('|1| a) ('|2| d))
     (cons a d))))
(define ::? pair?)
(define (::-content p)
  (make-immutable-hash-table
   (list (cons '|1| (car p))
         (cons '|2| (cdr p)))))

(define my-append
  (match-lambda
    ((hash-table ('|1| (? nil?)) ('|2| l)) l)
    ((hash-table
      ('|1| (? ::? (app ::-content (hash-table ('|1| h) ('|2| t)))))
      ('|2| l))
     (make-::
      (make-immutable-hash-table
       (list
        (cons '|1| h)
        (cons
         '|2|
         (my-append
          (make-immutable-hash-table
           (list (cons '|1| t) (cons '|2| l)))))))))))


;matchs fine
(match '(a b)
  ((? ::? (app ::-content (hash-table ('|1| h) ('|2| t))))
   (values h t)))

;matchs fine
(my-append (make-immutable-hash-table
            (list (cons '|1| '())
                  (cons '|2| '(1 2)))))


;match error, why?
(my-append (make-immutable-hash-table
            (list (cons '|1| '(a b))
                  (cons '|2| '(1 2)))))