Product Discount works on 1st product only?

I have a 'Calculate product discounts' CA set up for a product based on a user role. Works fine when you add this product to your cart, but when you add another of the same product, the discount is not applied (or perhaps the discount on the first product in the cart is lost)

• The product has attributes that need to be supplied by the user before it can be added to the cart
• The condition checks the user's role and the node's ID value
• The action applies a discount to he product (Add -20.00)
• Drupal 6.14, Ubercart 6.x-2.0-rc7, Discount Framework 6.x-1.x-dev

Any ideas?