custom post type single post issue

I am editing a theme that uses a custom post type that works fine but when I try to view the single post it returns “page not found”.

I named the single “single-jj_projects.php” as the codex suggests but have had no joy.

add_action( 'init', 'create_post_type_projects' );
    function create_post_type_projects() {
    register_post_type( 'jj_projects',
	array(
	'labels' => array(
	'name' => __( 'Projects' ),
	'singular_name' => __( 'Projects' )
	),
	'hierarchical' => true,
	'public' => true,
	'has_archive' => true,
	'show_in_menu' => true,
	'menu_position' => 5,
	'supports' => array( 'title', 'thumbnail', 'editor', 'excerpt'),
	'show_ui' => true,
	)
    );
}