Featured Image not working on some Posts

Hello,
new to the forum. I have a website I am working on that is having the following issue.
One of the posts in question is http://www.midwaytrailers.net/342/
On some of the posts the Featured Image is not displaying and is returning the code from the single.php (Single Post) file.
This is what is displayed in the post:
[custom_gallery style=”1″ source=”post=342″ link=”image” description=”1″ size=”100×75″ limit=”18″]

Below is a full copy of the code from single.php
‘
<?php
/* midwayts theme */
/* single.php: dynamic post template file */
/* Used: To build single post display */
/* Heirarchy: http://codex.wordpress.org/Template_Hierarchy */
?>

<? get_header(); ?>

<div id=”sidebar-left”>
<? include_once(“includes/sidenav-childcategories.php”); ?>
</div>

<div id=”content”>
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php $tour_custom_fields = get_post_custom(); ?>
<div class=”post”>
<div class=”product-content”>
“>Back to List
<h4><?php the_title(); ?></h4>
<?php
if (trim($tour_custom_fields[‘item_price’][‘0’]) != “”) { ?>
<span class=”attribute”>Price: <? echo $tour_custom_fields[‘item_price’][‘0’]; ?></span>
<?php }
if (trim($tour_custom_fields[‘item_number’][‘0’]) != “”) { ?>
<span class=”attribute”>Item Number: <? echo $tour_custom_fields[‘item_number’][‘0’]; ?></span>
<?php }
// now we need to get number of post images/thumbnails
$attachments = get_children(array(
‘post_parent’ => $post->ID,
‘post_status’ => ‘inherit’,
‘post_type’ => ‘attachment’,
‘post_mime_type’ => ‘image’,
‘numberposts’ => 5, // the number
));
$imgcount = 0;
foreach($attachments as $attachment_id => $attachment_info) { $imgcount++; $singleimgid = $attachment_id; }
// one or more images – display photo gallery
if (has_post_thumbnail() && $imgcount > 1) {
echo do_shortcode(‘[custom_gallery style=”1″ source=”post=’ . get_the_ID() . ‘” link=”image” description=”1″ size=”100×75″ limit=”18″]’);
// only single image – display featured image
} elseif (has_post_thumbnail() && $imgcount > 0) {
$large_image_url = wp_get_attachment_image_src($singleimgid,’large’);
echo ‘‘;
the_post_thumbnail(‘thumbnail’);
echo ‘‘;
} ?>
<?php the_content(); ?>
</div>
</div>
<?php endwhile; ?>
<?php else : ?>
<div class=”textcenter”><p class=”red”>Sorry, this listings is unavailable or has been removed, check back soon!</p></div>
<?php endif; ?>
<div class=”clear”></div>
</div>

<? get_footer(); ?>
‘