Checkout Button doesn't work

I checked demos of your templates. They look fine and pretty user-friendly and I was thinking about buying one of the templates that looks perfect for the online store I'm planning to set up. However, the problem is that in all your templates that I checked I was unable to finish the purchase. After I fill out the form to check out as a guest, I go to the final checkout page, press the checkout button, but nothing happens. I stay on the same checkout page and the cart still contains the product I was trying to buy.
Since this happens to me in all your virtuemart templates, I figure out the problem is on my side. But I wonder what the problem is. I tried different browsers and IPs. If you can explain it to me, I'll be happy to buy a paid membership.