ob_start() combined with wp_query

I'm making my first plugin and I have a problem with displaying my shortcode. It shows in the top all the time but I have read some about ob_start(); and trying to use it but the shortcode just returns nothing.

Im using the code below - it seems to have something to do with my posts. Anyone know why how to solve this problem?

My WP_Query:

$query = new WP_Query( array(
'category__in' => $categories,
'posts_per_page' => $whpost_stored_meta_shortcode['whpost_number_of_posts'][0]
  ));

The code i use to display it:

ob_start();

echo '<div class="whpost_content">';
    while($query->have_posts()) : $query->the_post();
      // Get the URL to the attached image.
      $attached_image = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), 'large' );
      echo '<div class="whpost_post" style="width: ' . $post_width .'; background-image: url(' . $attached_image[0] . ');">';
        the_title();
      echo '</div>';
    endwhile;
  echo '</div>';

  return ob_get_clean();

The complete code for the shortcode function:

  <?php
function cpt_content_func( $atts ) {
  // Get the ID we putted in into [wh-postgrid id="THIS_ID"]
  extract( shortcode_atts( array(
    'id' => null
  ), $atts ) );

  // Get stored meta data (For the categories - needed to be formatted in
  // a certain way)
  $categories = get_post_meta( $id, 'whpost_cats', true );

  // Get meta data for settings and such.
  $whpost_stored_meta_shortcode = get_post_meta( $id );

  // Get the correct categories and use the settings we got.
  $query = new WP_Query( array(
    'category__in' => $categories,
    'posts_per_page' => $whpost_stored_meta_shortcode['whpost_number_of_posts'][0]
  ));

  // Set the styles
  switch ( $whpost_stored_meta_shortcode['whpost_posts_per_line'][0] ) {
    case 1:
      $post_width = '100%';
      $post_max_height = '';
      break;

    case 2:
      $post_width = '50%';
      $post_max_height = '';
      break;

    case 3:
      $post_width = '33.333333%';
      $post_max_height = '';
      break;

    case 4:
      $post_width = '25%';
      $post_max_height = '';
      break;

    default:
      $post_width = '50%';
  }



  // Display the front-end
  ob_start();

  echo '<div class="whpost_content">';
    while($query->have_posts()) : $query->the_post();
      // Get the URL to the attached image.
      $attached_image = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), 'large' );
      echo '<div class="whpost_post" style="width: ' . $post_width .'; background-image: url(' . $attached_image[0] . ');">';
        the_title();
      echo '</div>';
    endwhile;
  echo '</div>';

  return ob_get_clean();
}
add_shortcode('wh-postgrid','cpt_content_func');

via Chebli Mohamed