Checkout with Paypal just refreshes the "choose desired plan" page?

Hi, I've setup Pro Sites for the first time. However, one of the key features is not working for me: when a user is prompted to renew or pay for their subscription to re-activate their site, they can select the month/3month/yr option, but on clicking the Checkout with PayPal button, the page just refreshes. They are not taken to PayPal. Any ideas what might cause this?