comment_parent not storing

I am in the process of creating a theme and some how have broken threaded comments. Having traced through the code I have checked up to this in a custom wp-comments-post file:

$comment_parent = isset($_POST['comment_parent']) ? absint($_POST['comment_parent']) : 0;
$commentdata = compact('comment_post_ID', 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_content', 'comment_type', 'comment_parent', 'user_ID');
print_r($commentdata);
$comment_id = wp_new_comment( $commentdata );

which outputs correctly:

Array ( [comment_post_ID] => 20 [comment_author] => Tim H [comment_author_email] => *deleted* [comment_author_url] => [comment_content] => tes4 [comment_type] => [comment_parent] => 8 [user_ID] => 0 )

however in the database comment_parent is zero ?