Parent Designer doesn't save to cart of child items

Hi I've set up a product with childs as colours to select from. The VDesigner is on the parent level custom fields. When you select a child variant on the main public page, you can design a design, save it and appears below the designer as a thumbnail. If you add to basket it doesn not save it to basket. If you copy to each child it works fine. Ideally having it work on the parent would be the easiest as it saves having to duplicate to every child/ alternatively is there a way to change the default values if setting on parent isn't possible?