the_post_thumbnail not working

I am attempting to use the_post_thumbnail tag on a main page template, and it does not work. The code below I’m using is a modified version from Pages – A Page of Posts

Can anyone tell me why the_post_thumbnail tag isn’t working? I’ve enabled post thumbnails by adding add_theme_support('post-thumbnails'); to the theme’s functions.php file, but still no thumbnail display.

Thanks

<?php if( $my_query->have_posts() ) : ?>
		<?php while ($my_query->have_posts()) : $my_query->the_post(); ?>
			<?php
			//necessary to show the tags
			global $wp_query;
			$wp_query->in_the_loop = true;
			?>
			<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
				<?php the_post_thumbnail(); ?>
				<h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
				<small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>
				<div class="entry">
					<?php the_excerpt(); ?>
				</div>
			</div>
		<?php endwhile; ?>

	<?php else : ?>
		<h2 class="center">Not Found</h2>
		<p class="center">Sorry, but you are looking for something that isn't here.</p>
		<?php get_search_form(); ?>
	<?php endif; ?>