Thumbnail issues in portfolio

I’m having an issue where I want the thumbnails on our portfolio page to come in at the size set for thumbnails in the media settings (174 x 126px) but it’s coming in at full size. I’ve remade the thumbnail and checked the uploads folder and can see that the thumbnail is the correct size. What am I missing?

Page is https://chorusdesign.com/our-work/

My portfolio page looks like this:

<?php
/* Template Name: Portfolio */
?>

<?php get_header(); ?>

<div id="main-content">

	<?php
		$loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 24));
	?>
	<?php while ( $loop->have_posts() ) : $loop->the_post(); ?>

	<?php
		$custom = get_post_custom($post->ID);
		$screenshot_url = $custom["screenshot_url"][0];
		$website_url = $custom["website_url"][0];
	?>

        <div id="portfolio-item">
		<a href="<?php the_permalink(); ?>"><?php the_post_thumbnail(); ?> </a>
		<h1><?php the_title(); ?></h1>
		</div>
        <?php endwhile; ?>
</div><!-- #content -->

<?php get_sidebar(); ?>

<?php get_footer(); ?>