It seems that you might be getting some errors or warnings on this page that you are not seeing when you click on the add to cart button. Please put this code on the page toward the top of the page:
This will enable error reporting for you so that we can see what errors might be occurring on the page. Please post back and let us know what you get after adding this to the page and testing the page. If the issue is not apparent or further troubleshooting needs to be done I can open a ticket for you on this issue.