Why isn’t wp_enqueue_script(‘jquery-masonry’) working?

jQuery’s Masonry is bundled with WordPress. All you should need is wp_enqueue_script('jquery-masonry'); but note that the “slug” is jquery-masonry, not jquery_masonry. And be sure your are enqueueing on the wp_enqueue_scripts hook or later.

function enqueue_masonry() {
  wp_enqueue_script('jquery-masonry');
}
add_action('wp_enqueue_scripts','enqueue_masonry');

I’d guess that your problem has something to do with when you register/enqueue because loading core scripts is pretty straightforward.

If that doesn’t work, something is wrong with your site. A plugin or a theme is deregistering, perhaps. Without more information it is going to be hard to say what.