Add-to-cart button disappeared from one product

Hi, two days ago in one of the products I have (a variable product) the add-to-cart button suddenly disappeared. When I select the product variation the button simply does not appear anymore.

I checked with another product, which is set exactly as the not working one, and everything works out perfectly.

I uregentely need to solve the problem. I checked in the woocommerce settings regarding possible limits to the quantity of a product that could be sold, but I found only that these are appied to downloadable products (which is not my case).

The product that is not working can be found here : http://www.tethys.org/tethys/products/csr-cat/csr-expeditions/

while the similar product which is working is here: http://www.tethys.org/tethys/products/idp-cat/idp-expeditions/

Thanks

https://wordpress.org/plugins/woocommerce/