Add to cart works…but doesn't

Hi there,

I can’t seem to get my “Add to cart” button to work perfectly normal. When the user clicks on it, the button shrinks then they have to click on it again before the item gets added to the cart. Any idea why?

Here is the link to the site: https://divachoice.co.uk/diva-royalty-collection/
I notice when i changed the theme, the button works perfectly fine.
WP e-Commerce Version 3.8.8.5 is the plugin im using.