next_posts_link not working on single.php

I am using a pretty simple single.php page and I am having a hard time getting the next_posts_link and previous_post_link to work correctly. I am using the basic single.php structure.

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<?php endwhile; else: ?>
<?php endif; ?>
<p class="small"><?php next_posts_link(); ?></p>
<p class="small"><?php previous_posts_link(); ?></p>

I have tried putting it inside the loop outside the loop, but nothing will get it to show up? Any ideas. thank you in advance.