I am testing a 100% coupon on my site to comp membership

I am testing a 100% coupon on my site to comp membership to people. I apply it and it works fine. However, I end up back at the register page instead of the custom thank you page I have set up for completion of purchase.

Furthermore, even though the user's been shifted to the right level of membership/access, they still see the menu items for non-subscribed/non-authenticated users instead of the member's menu.

It appears as if nothing has happened (but they do in fact have access to the parts of the site they should -- so it is an aesthetic issue). I hope that makes sense...