Variations no longer add to cart

My client has just contacted me to tell me her variable items no longer add to her shopping cart. Individual ones do. The site has been working without issue for a little while now. An example of a variable item can be seen here: https://www.middletonhair.com.au/product/refresh-shampoo/ I’m using version 2.5.2

Any ideas why this could have happened?

Thanks

Eddie

https://www.ads-software.com/plugins/woocommerce/