Virtuemart2 item not in cart

[ereallstaff]

Hello everybody, I know this is out of support, but it's a general technical problem. on our www.modacalcio.it , many customers says that after adding product to the cart , their cart is empty

the strange things is that when I ask them to make from mobile, they have same problem . Also from a different wireless ( I have seen different isp acting differently on web, so I don't want to exclude any reason )

What can be so the problem? Cookies ? session ?