Hello,
What plugin do you use to implement this feature? If you use Product Addons by WooCommerce then you need to disable Ajax Add to cart option in Theme Options > E-Commerce > Single Product > disable AJAX add to cart option because this plugin uses POST method to add to cart and does not work with Ajax.
To check if problem is related to our theme enable official WooCommerce theme – Storefront and check.
Provide also admin panel and FTP access if problem persists with our theme only.
Regards