Update post meta on bulk edit / update

I have a small problem with a code that works most of the time. I just discovered it cannot be used when I bulk edit custom post type.
If I’m on individual post page and hit publish, the custom post type gets published and the user meta gets updated.

If I use the bulk edit to publish many posts at once, the user meta is not updated.

    function sa_ads_count() {
        global $wpdb, $post;
        $authordata = get_userdata($post->post_author);
        $author = $authordata->ID;
        // get initial count
        $single = true;
        $ads_count = get_user_meta( $author, 'ads_count', $single );
        if ( strlen( $ads_count ) > 0 ) {
            $ads_count = intval( $ads_count ); // make sure it's a number
            $ads_count++; // increment by one
        } else {
            // meta isn't set, so we'll create it
            $rows = $wpdb->get_results( $wpdb->prepare( "SELECT post_status, COUNT(ID) as count FROM $wpdb->posts WHERE post_author = %d AND post_type = 'post_type' GROUP BY post_status", $author ) );

$stats = array();
foreach ( $rows as $row )
    $stats[ $row->post_status ] = $row->count;
            $ads_count = isset($stats['publish']) ? $stats['publish'] : 0;
        }

        update_user_meta( $author, 'ads_count', $ads_count );
    }

add_action( 'pending_to_publish', 'sa_ads_count' );

Any idea what is wrong here? Is it a WordPress limitation or this can be solved?
The thing is, I need this hook as I only want to count and store for every user, the number of posts that I approve and publish.

Later on, I want to change new post status directly to publish based on the number of posts the user has approved. This I already achieved.