Wordpress next/prev post on single custom post page? - wordpress

Question

So I've got a custom post type, all working fine.
Goes to single-portfolio, but it won't paginate.
I can't, no matter what I try get pagination to work on the single page.
Here's my code.
<?php get_header(); ?>
<div class="main-content">
<div class="portfoliopost">
<div class="col-full">
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
<div class="port-left-single">
<h2><?php the_title(); ?></h2>
<div class="projecttype">
<ul>
<?php
$posttags = get_the_tags();
if ($posttags) {
foreach($posttags as $tag) {
echo '<li>' .$tag->name. '</li>';
}
}
?>
</ul>
</div>
<?php the_field('main_portfolio_meta'); ?>
</div> <!-- end div port-left-single -->
<div class="port-right-single">
<?php the_content(); ?>
</div>
<div style="clear:both;"></div>
</div> <!-- end div post-class -->
<div class="navigation">
<div class="next-posts"><?php next_posts_link('Older Posts') ?></div>
<div class="prev-posts"><?php previous_posts_link('Newer Posts') ?></div>
</div>
<?php endwhile; endif; ?>
</div> <!-- end div col-full -->
</div> <!-- end div portfolipost -->
<div style="clear:both;"></div>
</div> <!-- end div main content -->
<?php get_footer(); ?>
It's inside the loop, so what gives?