zen cart version 1.3.8a with google checkout version 1.47

ok so just setup the settings in google checkout today (added merchant number and key) and took google checkout out of sandbox mode. went to try a test order to make sure it directs to google checkout without a problem and when i get to step 3 of 3 and get presented with the confirm order button it simply loads the shopping cart again??? I can only assume that any potential customers at this point would have completly lost the enthusiasm to order from me and have pressed close tab. any ideas what could be causing this error??

other modules installed in this heavily modified cart are.... cardsave iridium module, reward points, google analytics, simple seo url's (disabled as makes a mess of iridium module) fast and easy checkout, google analytics and a custom built mod by a developer on rent a coder that delivers 25 digit codes (xbox codes) by email upon purchase. just thought they might be worth a mention as they could also be causing the error. and also the reason i didnt make a post in the official thread for support on this particular occassion. (i felt it may confuse other members reading advice given to myself and cause mass hysteria for google checkout users that are members here)