product variation

i add a product_variation with this code:

$variation_post = array( // Setup the post data for the variation

            'post_title'  => 'Variation product#'. $parent_id,
            'post_name'   => 'product-'.$parent_id.'-variation-',
            'post_status' => 'publish',
            'post_parent' => $parent_id,
            'post_type'   => 'product_variation',
            'guid'        => home_url() . '/?product_variation=product-' . $parent_id . '-variation-'
        );
		// The variation id
		$variation_id = wp_insert_post( $variation_post );

and add some attributes and all is well in the admin but in the front the product not showing the
variation with attribute untill i hit update then it shows up

any clue?