I think this may be related to your issue. From what you describe, it's very similar to my environment. And I found a bug. I posted the following in a couple other threads.

I've had this same problem intermittently for a while. I've been through the forums here for help but just found that lots of us have the issue. So I decided to debug myself. Likewise for me, all works well in my dev environment, but not on QA and production. I believe I've found the issue and I've put in a bug report. I'd bet in your dev environment you run your database server on the same machine as your web server and in your live environment they are on separate machines. Take a look at http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=7070&PROJID=23. I put my workaround in there. If that doesn't work for you, after you understand the problem, you'll probably be able to come up with another.

One final note: You still need to clear the "portalroles" cookie and the user cache sometime between adding the new role and redirecting to a page that requires the new role. The following two lines of code will do it:

         DotNetNuke.Security.PortalSecurity.ClearRoles();
         DataCache.ClearUserCache(PortalId, Username);