Thumbnails on next/previous links in custom post type single.php

Question

I've been successful in using the following code to display thumbnails for next & previous links for normal posts but this does not work at all for custom post types.

<div id="cooler-nav" class="navigation">
                <?php $prevPost = get_previous_post(true);
                if($prevPost) {?>
                <div class="nav-box previous">
                    <?php $prevthumbnail = get_the_post_thumbnail($prevPost->ID, 'tiny_thumb' );?>
                    <?php previous_post_link('%link',"$prevthumbnail  <p>%title</p>", TRUE); ?>
                </div>

                <?php } $nextPost = get_next_post(true);
                if($nextPost) { ?>
                <div class="nav-box next">
                    <?php $nextthumbnail = get_the_post_thumbnail($nextPost->ID, 'tiny_thumb' ); } ?>
                    <?php next_post_link('%link',"$nextthumbnail  <p>%title</p>", TRUE); ?>
                </div>
                <?php  ?>
            </div><!--#cooler-nav div -->

I'm guessing it must be a special query to look for my custom post type "portfolio"

Any help would be greatly appreciated

EDIT

<nav class="clearer">

                <?php if( $prev_post = get_previous_post() ): ?>
                    <?php echo $prev_post->ID ?>
                <div class="nav-box previous">
                    <?php $prevthumbnail = get_the_post_thumbnail($prev_post->ID, 'tiny_thumb' );?>
                    <?php previous_post_link('%link',"$prevthumbnail  <p>%title</p>", TRUE); ?>
                </div>
                <?php endif; ?>

                <?php if( $next_post = get_next_post() ): ?>
                    <?php echo $next_post->ID ?>
                    <div class="nav-box next">
                        <?php $nextthumbnail = get_the_post_thumbnail($next_post->ID, 'tiny_thumb' );  ?>
                        <?php next_post_link('%link',"$nextthumbnail  <p>%title</p>", TRUE); ?>
                    </div>
                <?php endif; ?>

            </nav>

Other loop not working (I am getting thumbnails & links but no exclusion of current post or randomizing of order)

<?php   
                    $t_args = array (
                       'tax_query' => array (
                          array (
                            'orderby' => 'rand',
                            'post_type' => 'portfolio',
                            'taxonomy' => 'service',
                            'field' => 'slug',
                            'terms' => $service_type,
                            'post__not_in' => array($post->ID)

                          )
                       )
                    );

                    $query = new WP_Query( $t_args );

                    if( $query->have_posts() ) {
                       while ( $query->have_posts() ) : $query->the_post();

                          // output your stuff
                          echo '<div class="more_from_thumb"><a href="';
                          the_permalink();
                          echo '">';
                          the_post_thumbnail();
                          echo '</a></div>';
                       endwhile;
                       wp_reset_query();
                    } ?>

Answer 1

The first argument that get_previous_post and get_next_post is $in_same_cat. WordPress is looking for post of the same type in the current posts category. If your custom post type doesn't support the category taxonomy, both functions are likely to return nothing for the previous and next post.

Try calling both functions without arguments for your custom post type.

<?php if( $prev_post = get_previous_post() ): ?>
    <div class="nav-box previous">
        <?php $prevthumbnail = get_the_post_thumbnail($prev_post->ID, 'tiny_thumb' );?>
        <?php previous_post_link('%link',"$prevthumbnail  <p>%title</p>", TRUE); ?>
    </div>
<?php endif; ?>

<?php if( $next_post = get_next_post() ): ?>
    <div class="nav-box next">
        <?php $nextthumbnail = get_the_post_thumbnail($next_post->ID, 'tiny_thumb' ); } ?>
        <?php next_post_link('%link',"$nextthumbnail  <p>%title</p>", TRUE); ?>
    </div>
<?php endif; ?>