Hi,
With the new add to cart system for the cart moduel to be updated, it requires some additional javascript code which wasn't there before.
What this means is that if you had previously customized the file "cart" of the view "product", which displays the cart module, you're missing that javascript in the cart module and thus the cart module is not linked to the new add to cart ajax system.
So you would have to delete the customization of that view file via the menu Display>Views and reapply your changes to the new version of the file and then the cart module should update dynamically when you click on the new add to cart buttons.