Problem sorting blog posts by postCommentCount

I am trying to created a trending blog posts area on the sidebar of my blog. I am trying to sort the blog posts by comment count, from most comments to least. Below is the code on my blog page to sort by postCommentCount:

                        <?php 
                            perch_blog_custom(array(
                            'template' => 'post_in_list_trending.html',
                            'sort' => 'postCommentCount',
                            'sort-order' => 'DESC',
                            'start' => '1',
                            'count' => '3',
                            ));
                        ?>

It seemed to work correctly until I added more comments to blog posts and the count seems to not be updating correctly.

On the page_in_list.html template, I am using

<perch:blog id="postCommentCount"/>

and the count is updating and displaying correctly. The template for post_in_list_trending.html is below:

<div class="trending_post">
        <a href="<perch:blog id="postURL" />"><img src="<perch:blog id="blog_image" type="image" width="220" height="300" crop="true" />" alt="<perch:blog id="postTitle" />" /></a>
        <div class="trending_ribbon">
            <p><a href="<perch:blog id="postURL" />"><perch:blog id="postTitle" /> <i class="icon-comment-1"></i> <perch:blog id="postCommentCount"/></a></p>
        </div>          
</div>

Any ideas on why this isn't working correctly? Thanks