Subscription content doesn't show up on main page

I've created a page - http://www.12kpost.com/purchase/ - that when you go to that page it shows the purchase buttons and content that I've entered using the [subscriptionform] shortcode. When I add that page to the main menu, the content is no longer available - http://www.12kpost.com/?menu=sign-up

Can you provide some insight into why the content shows on one page and not the other?

Thank you,

Tyler