Taxonomy Query

I have a single custom post type of ‘products’ while that cpt is organized by 5 taxonomy categories which include ‘equipment’, ‘tools’ and ‘supplies’.

On my single-equipment.php page (single custom post type template page) I would like to show ‘related-items’ where the page pulls other items from the related taxonomy shown on the given page. In a sense, it’s basically a “see more items like this one” type of thing.

So let’s say a viewer is looking at a page for ‘window cleaner fluid’ that belongs to the category/taxonomy of ‘supplies’, I want to have 3 or 4 items pull from the ‘supplies’ taxonomy to show at the bottom of the post before the footer.

So far, I have the following:

function smt_related_items() {

  $terms = get_the_terms( get_the_ID());
  $args = array(
    'posts_per_page' => 4,
    'post__not_in' => array( get_the_ID() ),
    'category_name' => $terms[0]->slug
  );
  $loop = new WP_Query( $args );

  if( $loop->have_posts() ):
    echo '<div class="related-items"><h3>Related Items from Summit</h3>';
    while( $loop->have_posts() ): $loop->the_post();
      $classes = 'one-half relate-box';
      if( 0 == $loop->current_post || 0 == $loop->current_post % 2 )
        $classes .= ' first';
      echo '<div class="' . $classes . '"><a href="' . get_permalink() . '">' . get_the_post_thumbnail( get_the_ID(), 'pdt-thumb' ) . '</a><p>' . get_the_excerpt() .'</p></div>';
    endwhile;
    echo '</div>';
  endif;
  wp_reset_postdata();

}

but my page simply seems to only return 4 posts from wherever on the site, including from the normal posts categories with no distinguishing on where it’s pulling posts from. I’ve been stuck on this for like 3 days, and I can’t figure it out.

Can anyone help!?