自定义帖子类型的Wordpress查询帖子不起作用

Question

I'm trying to display the featured images of my custom post type of 'instrument'

I created the custom post type using the plugin 'Custom Post Type UI'

The query seems to work, but the problem is that it does not seem to loop - only the most recent post is displayed.

However, when I try this same code but querying all posts with a category of instruments, normal posts with the category will show, but not my custom posts with the category added.

This is the code:

         <?php query_posts('post_type=instrument&posts_per_page=3');
               while (have_posts()) : the_post();?>

                                <div class="col-sm-2">

                                    <a href="<?php echo get_permalink(); ?>">   

                                        <?php the_post_thumbnail('medium'); ?>

                                    </a>

                                </div><!--.col-sm-2-->

           <?php endwhile; ?>