You probably need to add <?php session_start(); ?> to the top of your add to cart and shopping cart pages. Since you are using a cookie cart the session may not be started automatically... and the previous page setting always uses sessions.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.