Post Navigation Help

Hi, my theme is not displaying a 'previous post' or 'next post' link
I believe the code for it (although slightly different) is in the single post loop already

<?php the_content( __('<p>Read the rest of this entry &raquo;</p>', 'arras') ); ?>
<?php wp_link_pages(array('before' => __('<p><strong>Pages:</strong> ', 'arras'),
'after' => '</p>', 'next_or_number' => 'number'));

I may be wrong of course, but even if i do enter the code <?php previous_posts_link(); ?>
<?php next_posts_link(); ?>
into the loop or even in individual post's i get nothing. It seems my theme just ignores the code, as if i have a setting wrong somewhere?
Any help much appreciated

I don't know what you are going do to but:

wp_link_pages() should be used only for paginated posts (posts with <!--nextpage--> quicktag)

previous_posts_link() and next_posts_link() are relative to a query, so they have to be placed outside the loop.