Since WordPress is having trouble recognizing your custom template files, you could use this instead.
Insert into your single.php file. Based on your description above, it sounds like you don’t have a single.php file so you will likely have to create one. If you do already have a single.php file then there is likely still some confusion with your exact problem because a custom post would only display the index.php template if there was no single.php. Or you could just be confusing the two.
<?php
if( get_post_type == 'acme_product' ) {
// Put the template code to display the custom post type post
} else {
// Code for all other posts
}
?>
This will work in any of the default WordPress post related template files like categories, archives, etc.