An AJAX function doesn't recognize mi plugins

I have this function on functions.php on my theme:

add_action('wp_ajax_get_single_post', 'single_post_callback');
function single_post_callback(){
if (isset($_POST['postName'])) {
$postName = $_POST['postName'];
$queryargs = array('showposts' => 1, 'name' => $postName);
$myquery = new WP_Query($queryargs);
if ($myquery->have_posts()) {
while ($myquery->have_posts()) : $myquery->the_post();
 ?> <h2> <?php the_title(); ?> </h2> <?php
the_content();
endwhile;
}
else {
	echo "No post with name " . $postName;
}

die();
}
else echo "No postName";
}

The problem is that the_content() function show this when I have shortcodes on my post instead of the plugin functionality that mus display:

[download#15]

[contact-form 6 "Name of form"]

Any ideas?