I'm working on a custom page for a client that when they click on a product on this page, it sends them to signin.aspx?returnurl=showproduct.aspx?productid=# ###. This is working like a charm on our development site, but when we uploaded these changes to the live site, it would return the user to the proper URL, but the user would appear as if they weren't signed in. They're getting the "must be signed in to view this product" text. On this page, clicking on any other link which goes to another shop page, the user then appears signed in and can view products as they should be able to.

The only difference between our dev & production site is that production is using SSL whereas dev is not. Any ideas why this might be happening?