I have created the following hook in my functions.php

add_filter('wp_insert_post_data', 'ad_change_title');
function ad_change_title($data)
{
    if($data['post_type'] != 'ad_listing')
        return $data;
    if(!is_admin())
        return $data;

    global $post;   
    update_post_meta((int)$post->ID, 'cp_entitle_englishel', (string)$_POST['qtrans_title_en']);

    return $data;
}

but the update_post_meta part. I have checked the post_id, cp_entitle_englishel, qtrans_title_en values, and they are valid. Any suggestions?