Only Serving Original Image Size

Hello! I have got this working great, but for some reason, it’s showing me only the original image size and not the ‘medium’ size I have assigned it. I’ve tried thumbnail. I’ve tried an array. I’ve tried all the sizes. It only shows the original image. Any ideas?

// 		echo '<div id="author-photos">';
		foreach (get_the_terms(get_the_ID(), 'book_author') as $cat) :
			echo '<div class="author-photo">';
			// author photo with link
			echo '<a href="'
				. get_term_link($cat->term_id, 'book_author') .
			'"><img src="'
				. z_taxonomy_image_url($cat->term_id, 'medium') .
			'" /></a>';
	
			//author name with link
			echo '<a href="'
				. get_term_link($cat->term_id, 'book_author') .
			'"><span>';
			echo $cat->name;
			echo '</span></a>';
			echo '</div>';
		endforeach;
		echo '</div>'; // end author photos