Delete your browser cookies to resolve this problem. Sometimes when you are adding or editing products and other settings in Seller Admin while also testing your purchase buttons for a buyer cart at the same time, the cookie we set in your browser to track your cart contents can get confused, so only the current item added to a cart will show up in the cart, and any previously-added items will seem to disappear from your cart. Clearing the cookies in your browser will fix this problem for you.