There’s a limit set inside the function that lists the meta data.
http://core.trac.wordpress.org/browser/tags/3.1/wp-admin/includes/template.php#L512
Run a filter on postmeta_form_limit to increase to your desired value, eg.
add_filter( 'postmeta_form_limit', 'meta_limit_increase' );
function meta_limit_increase( $limit ) {
return 50;
}
Hope that helps.. 🙂