Sub-stores add to cart does not add anything to cart

My main site ( has subsites (for example, When I go to a subsite, and add an item to the cart, nothing is added to the cart. Nothing is also added to the "global cart". It seems like the carts in general aren't hooked up to one another. Seems like there are multiple problems involved. So there must be something going on. Can you please help?
Thank you,
David A.