Problem with WP_Query and Search Parameter

Hello,

I have a problem with WP_Query and Search Parameter.
Following my code:

$selection = array(
‘post_type’ => $posttype,
‘s’ => $search,
‘posts_per_page’ => 10
);

I get the value of $search by a GET[].
And below my loop.

<?php
$query = new WP_Query( $selection );?>
<?php while ( $query->have_posts() ) : $query->the_post(); ?>
<h2><?php the_title(); ?></h2>
<?php endwhile; ?>

My problem is that I NOT obtain any receipts if $search equal something and I am sure that I should have 3 receipts.

Does anyone has an idea? Should I change my search.php? I use the standard template “twentythirteen”.
Does anyone know how to use Search parameter?

Thanks in advance.
Best regards,
Gwen.