Checkout Problem @ Delivery Details [Opencart 1.5.1.3]

I am using Opencart 1.5.1.3 At checkout page, on the 3rd step continue button doesn't work.. But if you only click fast and continiously it passes to next step.. Does any one who knows what can be cause this and can fix this kind of problems? If there are freelancer friends interested in this and 2 other opencart sites' maintainance works please contact. Thanks in advance.. (I am very very new user)