short code wont show posts

I am trying to get custom post types in my plugin and then show them
on a static page with the short code [forum]. The function works “inside” wordpress when im trying it in a template but not when im running it in my plugin file and then activating it with add_shortcode( ‘forum’, ‘fLoop’ ).

What could be wrong here?

function fLoop(){

   $mypost = array( 'post_type' => 'forum_custompost', );
    $loop = new WP_Query( $mypost );

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

     $output = '<div class="clear"></div><div class="childs grid_12">';
        $output .=
                   '<div class="entry-content"'.
                   the_content().'</div>';
    endwhile;
    $output .= '</div>';
return  $output ;
}

add_shortcode( 'forum', 'fLoop' );