next / previous posts link in a single page

I’ve read the other posts in the forum but i can’t solve the problem.

I’m trying to display a single posts in a single page with the navigation “next_post_link” and “previous_post_link()”. But it seems not to work properly.

This is the code i wrote

<?php if (is_single()) : ?>
<div id="navigation">
<?php previous_post_link('%link', 'Previous post in category', TRUE, ''); ?> | <?php next_post_link('%link', 'Next post in category', TRUE, ''); ?></div>
<?php endif; ?>

It doesn’t display the previuos link and the next link stops the navigation at the 2nd post.

Can anybody help?
Thanks
Alberto