Problem with co-author bio box below article

I am managing a site using the 15Zine Theme. It automatically displays an author bio on single.php after the post content. Some of the articles have more than one author. I use the Co-authors Plus plugin to show all of them. At the moment, all authors are shown in the byline, but not at the bottom of the post. The bio only shows the first author. How can I get two author bios to show up when two authors are listed?

In the single.php the code says:

<footer class="cb-article-footer">
    <?php
	if ( ot_get_option('cb_tags_onoff', 'on') != 'off' ) { the_tags('<p class="cb-tags cb-post-footer-block"> ', '', '</p>'); }
echo cb_sharing_block( $post );
echo cb_post_footer_ad();
if ( $post->post_type != 'attachment' ) { cb_previous_next_links(); }
echo cb_about_author( $post );
cb_related_posts();
comments_template();
?>
</footer> <!-- end article footer -->

Many thanks in advance!