I have found Product Owners disinterested, almost, when we ask them for priorities at a story card level. This is usually because its too low-level. Its more important for the Product Owner to first prioritize at a theme level.
Whats a theme? A theme is a business process that can later be broken into story cards. For example a theme for a retailer wanting to go online might be “A user can checkout a product on my site”. This theme can be broken into dozens of stories, “Add a product to cart”, “Checkout a product”, “Provide Shipping and Billing Address”, “Provide Billing Information”, “Google Checkout”, “Paypal”, “Visa”, “Mastercard”, “Bill Me Later”, “Save profile”.
Start with a list of themes and arrange them in the order as above. Then start with the mandatory themes and break those down into stories. Stories can then be ordered the exact same way as above. Mike Cohn’s presentation at Agile 2008 explains the way themes can be bucketed really well.
Note: A theme is not the same as an epic. An epic is really a large story. The advantage of this approach is that its very clear which themes are important and which stories within those themes. There is no risk of stories slipping through.