A common problem with Wp E-Commerce is with the Checkout page “Make Payment” button. Whenever users try to make a purchase it refreshes the page each time and goes no where. I’ve seen this problem with Google and Paypal Payment gateways. The problem seems to be a common one, this worked for us, maybe this will work for you.
Solution is to check-over the shipping settings:
If any of the shipping calculators are selected, deselect all of them and then enable only one at a time. One problem I noticed is with the weight rate. If the weight rate is not setup but is selected then disable it. Try using flat rate and put some value in it just for testing. After each change refresh the page and try to make a payment. That worked for us.
Also disable all the mandatory fields under the checkout settings of the Wp E-commerce plugin, you can enable then back after testing.