Hi,
I tried your website again, and it seems to be working like a charm.
I have added a product to the cart, then pressed "continue shopping", the item stayed in the cart and I go back to the previous page correctly.
It's possible that the way you are redirecting to the previous page influence the shopping cart, if you use a javascript.history.back it will go in the history of your browser and load a cart not updated, the best way is to save the url in session, and then redirect to this url when clicking on "continue shopping".