Displaying media for custom post type

I created a custom post type for Products. I can display all the fields no problem (either in a list or a single-product.php). I wanted to use the Media library and associate an uploaded picture with the Product post type (mostly to handle all the thumbnail work automatically). I uploaded a photo to the media library and associated with a specific Product post type.

 $args = array(
        'label' => __('Products'),
        'labels' => $labels,
        'public' => true,
        'can_export' => true,
        'show_ui' => true,
        '_builtin' => false,
        'capability_type' => 'post',
        'menu_icon' => get_bloginfo('template_url').'/functions/images/product.png',
        'hierarchical' => false,
        'rewrite' => array( "slug" => "product" ),
        'supports' => array('title', 'thumbnail'), //MAYBE add thumbnail later!
        'show_in_nav_menus' => true

        );

I’ve tried everything to display the thumbnail and picture for the Product but it’s not working: the_post_thumbnail();, get_the_post_thumbnail but nothing works. Here’s my single-product.php:

      <div class="post_content">
        <?php 
            $attachment_args = array(
                'post_type' => 'attachment',
                'numberposts' => -1,
                'post_status' => null,
                'post_parent' => $post->ID
                    );

            $attachment = get_posts($attachment_args); ?>
        <img src="<?php echo wp_get_attachment_image_src($attachment->ID, 'full', false); ?>"  />


        <p><?php echo get_post_meta($post->ID, $prefix.'description', true); ?></p>
        <ul>
        <?php //this gets the custom fields for the document links (PDF Files)
          $custom_field_keys = get_post_custom_keys();
          foreach ( $custom_field_keys as $key => $value ) {
            $valuet = trim($value);
              if ( '_' == $valuet{0} || $prefix != substr($valuet, 0, 4) || $prefix.'description' == $valuet )
              continue;
            $n = substr($valuet, -1);

            if ( $valuet == $prefix.'docname'.$n)
                echo '<li><a href="' . get_post_meta($post->ID, $prefix.'docurl'.$n, true) . '" target="_blank">' . get_post_meta($post->ID, $prefix.'docname'.$n, true) . '</a></li>';     
          }
        ?>
        </ul>
    </div><!-- end post_content -->

Am I missing something? Thank you!