Theme Thumbnails Support

Hi
I have developed a very simple WordPress theme but I have problem with thumbnails or featured image for my custom post types. I have added below codes to my function.php file for theme support of thumbnails, but yet have problem with the custom post type’s thumbnails. what is the problem?
Thanks

add_theme_support('post-thumbnails');

and also

function custom_theme_setup() {
    add_theme_support( 'post-thumbnails', array('portfolio'));
}
add_action( 'after_setup_theme', 'custom_theme_setup' );