WP e-commerce Plugin Checkout

I have WP e-commerce version 3.8.3 installed on WordPress and although everything works fine, the checkout does not work. I am using the manual payment option for bank transfers and when I go to the Checkout, My shopping cart shows fine, I enter the email and all other details, but when I click on “Purchase” the page just refreshes and the cart still shows and no order is recorded and no emails are sent to admin or the buyer. I also have the same issue on another site which previously worked fine. What went wrong and how do I fix it??