comment_reply_link after each child-comment?

I wonder why the comment_reply_link is only present after the main comment but not after each child comment. I wrote a callback function for wp_list_comments but I don’t understand the mechanics how I can place the reply-link after each child comment.

Any idea?

function comment_callback($comment, $args, $depth) {
   $GLOBALS['comment'] = $comment; ?>
   <li <?php comment_class(); ?> id="li-comment-<?php comment_ID() ?>">
     <div id="comment-<?php comment_ID(); ?>">
      <div class="comment-author vcard">
         <?php echo get_avatar($comment,$size='80',$default='http://www.gravatar.com/avatar/ad516503a11cd5ca435acc9bb6523536?s=80' ); ?>

         <?php printf(__('<cite class="fn">%s</cite> <span class="says">says:</span>'), get_comment_author_link()) ?>
      </div>
      <?php if ($comment->comment_approved == '0') : ?>
         <em><?php _e('Your comment is awaiting moderation.') ?></em>
         <br />
      <?php endif; ?>

      <div class="comment-meta commentmetadata"><a href="<?php echo htmlspecialchars( get_comment_link( $comment->comment_ID ) ) ?>"><?php printf(__('%1$s'), get_comment_date()) ?></a><?php edit_comment_link(__('(Edit)'),'  ','') ?></div>

      <?php comment_text() ?>

      <div class="reply">
         <?php comment_reply_link(array_merge( $args, array('depth' => $depth, 'max_depth' => $args['max_depth']))) ?>
      </div>
     </div>
<?php
}