WP_Query and displaying pages

Hello

I am having some difficulty displaying pages using WP_Query. My code is in functions.php as a part of a short code the following does not display the page when called.

function entertainment_monthly_listings_function($atts){

	$featured_winners_loop = new WP_Query( 'pagename=july-entertainment'); 

	while ( $featured_winners_loop->have_posts() ) : $featured_winners_loop->the_post(); 

                $return_string = the_content();

					endwhile;
					wp_reset_query();

   return $return_string;
}

but this does

function entertainment_monthly_listings_function($atts){

	$featured_winners_loop = new WP_Query( 'page_id=515'); 

	while ( $featured_winners_loop->have_posts() ) : $featured_winners_loop->the_post(); 

                $return_string = the_content();

					endwhile;
					wp_reset_query();

   return $return_string;
}

any help would be appreciated thanks