First, make sure you're typing in your correct billing information and card details - it's easy to forget whether the bank has your home or term-time address. Make sure you enter the address you registered your card to, and not just where you happen to be living now!
If that doesn't solve the problem, send us an email at website@guild.bham.ac.uk and we'll do our best to help you out. You'll make it easy for us to help you if you include your full name, student number AND a screenshot of your problem.