Cannot retrieve post

Hi,
I’ve create some indexes with custom fields but for one of the index I add the tags attached to the posts. Unfortunatly when I do a search I ca’t retrive the post with the tag. Any Ideas?

in my functions :

$tags = get_the_terms($post->ID, 'post_tag');
    $tagName = array();
    if ($tags) {
      foreach ($tags as $tag) {
        $tagName[] = $tag->name;
      }
    }
 $index['post_tag'] = implode($tagName);