featured image problem with custom post type

I would start by checking that everything is good with the image being saved to the CPT. Inside the loop…

<?php $url = wp_get_attachment_url( get_post_thumbnail_id($post->ID) ); ?>
<?php echo $url ?>

This will hopefully echo the image’s URL… if that is working then you probably have a problem with the loop conflicting with something else on the page.