Trackbacks not displaying

I’m building a new theme and I’m just working through the Theme Unit Test requirements in the Codex. I’m stuck on the Many Trackbacks test. My theme doesn’t show any Trackbacks but for the test post but when I switch to Twenty Eleven they appear. The comment count is correct.

Here’s what I have in comments.php:

<?php $comment_list_args = array('style' => 'div'); ?>
<div class="commentlist">
    <?php wp_list_comments($comment_list_args); ?>
</div>
<?php $comments_args = array(
            'title_reply' => __('<h4>Leave a Comment</h4>', 'mytheme'),
            'comment_notes_after' => '',
            'comment_field' => '<p class="comment-form-comment"><label for="comment">' . _x('Comment', 'noun', 'mytheme') . '</label><textarea id="comment" name="comment" aria-required="true"></textarea></p>',
            'style' => 'div',
        ); ?>
<div class="commentform">
    <?php comment_form($comments_args); ?>
</div>
<?php $pagination_args = array(
            'prev_text' => __('&lsaquo; Older', 'mytheme'), 
            'next_text' => __('Newer &rsaquo;', 'mytheme')
        ); ?>
<div class="pagination">
    <?php paginate_comments_links($pagination_args); ?>
</div>

I’ve tried adding 'type' => 'all' to the arguments for wp_list_comments but I get the same result.

All help much appreciated!