calling add_to_cart doesn’t add addon fields

We bought Product addons and it works fine. If I click on add to basket button it adds the product and the addons data fine. However, I also have an extra button where I “create” a product. It adds the first product to the basket then goes to a listing page to look for another product.

The problem is I can add to the basket but it doesn’t add the addons data – is there an extra call i need to make? I’m adding the product to the cart by calling:

global $woocommerce;
$woocommerce->cart->add_to_cart( $product_id );

Thanks