You may need to reset the design for completed orders and send the link for customization once again to your customer. You can make it via the Allow customers to edit again button in the order details page in your Zakeke back-office (see below).
This button is displayed when the status is Complete, as shown below.
After clicking on Allow customers to edit again the order returns to the status NEW and you will see only the button Show in the designer (see below)
Now it's possible to send the link to customize the design again to your customer.