wp_update_attachment_metadata not creating the other image sizes

I am trying to update an image attachment post and I’ve finally got it working to replace the image but wp_update_attachment_metadata does not create the thumbnails and the other media sizes. I’ve looked all over for a possible answer but to no avail.

Here a simplified version of the code that I am using which is basically the one provided in the wp_insert_attachment codex page:

$filename = 'http://www.mysite.com/wp-content/uploads/2012/01/Earth-Western-Hemisphere.jpg';

		$wp_filetype = wp_check_filetype(basename($filename), null );
		$attachment = array(
			'ID' => 705,
			'post_mime_type' => $wp_filetype['type'],
			'post_title' => preg_replace('/\.[^.]+$/', '', basename($filename)),
			'post_content' => '',
			'post_excerpt' => '',
			'post_status' => 'inherit'
		);
		$attach_id = wp_insert_attachment( $attachment, $filename, 704 );
		// you must first include the image.php file
		// for the function wp_generate_attachment_metadata() to work
		require_once(ABSPATH . 'wp-admin/includes/image.php');
		//update_attached_file( $attach_id, $filename );
		$attach_data = wp_generate_attachment_metadata( $attach_id, $filename );
		wp_update_attachment_metadata( $attach_id, $attach_data );

Any help would be greatly appreciated!

Thank you,
Anton