Since your add to cart functionality is on the detail page you will also need to have the validations occur on this page as well. The server validations should only be applied to the page that is specified in the action of the form since this is the page that receives the posted data from the form. Since this data is not being posted to the cart display page the validation that checks for it will fail every time. Please give this a try and if there are any further issues please let us know.