posts of same category in next_posts_link NOT working

I try to display Next/Previous link of posts in the same category, I follow the instruction of Template_Tags/next_post_link, but not working, I can’t figure out why. Here is my code in single.php:

<div class="navigation">
			<span class="pre-post"><?php previous_post_link('&laquo; %link','%title', true) ?></span>
			<span class="next-post"><?php next_post_link(' %link &raquo;','%title', true) ?></span>
		</div><!-- End#div.navigation -->

For more information, here is the whole code of the single.php:

<?php get_header(); ?>

<div id="content">

	<?php if(have_posts()) : ?>
		<?php while(have_posts()) : the_post(); ?>
			<div class="post" id="post-<?php the_ID(); ?>">
				<h2><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></h2>
				<div class="entry">
					<?php the_content(); ?>
					<?php link_pages(__('<p><strong>Pages:</strong> ','less_theme'), '</p>', 'number'); ?>
				</div><!-- End#div.entry -->
				<div class="comments-template">
					<?php comments_template('', true); ?>
				</div><!-- End#div.comments-template -->
			</div><!-- End#div.post-ID -->
		<div class="navigation">
			<span class="pre-post"><?php previous_post_link('&laquo; %link','%title', true) ?></span>
			<span class="next-post"><?php next_post_link(' %link &raquo;','%title', true) ?></span>
		</div><!-- End#div.navigation -->
		<?php endwhile; ?>
	<?php else : ?>
		<div class="post">
			<h2><?php _e('Not Found','less_theme'); ?></h2>
		</div><!-- End#div.post -->
	<?php endif; ?>

</div><!-- End#div.container -->

<?php get_sidebar(); ?>

<?php get_footer(); ?>