Unable to make tweetbacks work

Hi,

I don’t know what I am doing wrong but I don’t see any tweets in my comments.

1) Plugin is up to date and actived.
2) <?php comments_template('',true); ?> is set in single.php
3) Here my comments.php :

<?php
// Do not delete these lines
	if (!empty($_SERVER['SCRIPT_FILENAME']) &amp;&amp; 'comments.php' == basename($_SERVER['SCRIPT_FILENAME']))
		die ('Merci de ne pas lancer cette page directement');

	if ( post_password_required() ) { ?>
		<p class="nocomments">Cet article est protégé par un mot de passe. Entrez ce mot de passe pour lire les commentaires.</p>
	<?php
		return;
	}
?>

<!-- You can start editing here. -->
<div class="notice2">
</div>

<p>
<div class="commentnb">
<h2>
<?php comments_number(__('No Comments'), __('1 Comment'), __('% Comments')); ?>
<?php if ( comments_open() ) : ?>
</h2>
</div>
</p>
<?php if ( $comments ) : ?>
<?php $i = 0; ?>
<?php foreach ($comments as $comment) : ?>
<?php
	$comment_type = get_comment_type();
	if($comment_type == 'comment') {
?>
<?php $i++; ?>
	<li id="comment-<?php comment_ID() ?>"<?php if ($comment->comment_author_email == "[email protected]") echo ' class="author"'; else if($i&amp;1) { echo ' class="odd"';} else {echo ' class="even"';} ?>>

	<div class="commentimg">
	<?php echo get_avatar( $comment, 80 ); ?>
	<p><h4><?php comment_date() ?></h4></p>
	<p><h3><a href="#comment-<?php comment_ID() ?>"><?php comment_time() ?></a></h3></p>
	<p><h4><?php edit_comment_link(__("Edit This"), ''); ?></h4></p>
	</div>
	<div class="commentext">
	<h3><?php comment_author_link() ?></h3>
	<?php comment_text() ?>
	</div>
	<div class="clear">
	</div>
	</li>
<?php } ?>
<?php endforeach; ?>
<div class="notice2">
</div>
<p>
<div class="commentnb">
<h2>
Tweets
</h2>
</div>
</p>
<?php if ( ! empty($comments_by_type['tweetback']) ) : ?>
  <ol class="commentlist">
  <?php wp_list_comments('type=tweetback'); ?>
  </ol>
<?php endif; ?>
<div class="notice2">
</div>
<p>
<div class="commentnb">
<h2>
Rétroliens
</h2>
</div>
</p>
<?php if ( ! empty($comments_by_type['pings']) ) : ?>
  <ol class="commentlist">
  <?php wp_list_comments('type=pings'); ?>
  </ol>
<?php endif; ?>
<?php endif; ?>
<?php endif; ?>

Many thanks for your time and help