Aha, it looks like you're using the Ultimate Google Analytics plugin with your site, which is known to cause a conflict with our cart button code:
http://www.e-junkie.com/ej/trouble.site-blog.wordpress.htm
The way that plugin works, it rewrites a critical part of our button code that makes the button use our nice overlay-style cart "inside" your page, so the buttons affected by that conflict are operating in "fallback" mode as I described above. Add to Cart button codes for products using Variants/Variations are not affected by this, so they are still able to use the overlay cart.
Uninstalling the Ultimate GA plugin should fix the problem for you, or if you'd prefer to keep using that plugin, you can use the non-JavaScript version of our button codes -- just un-check the box indicated on our button codes screen to obtain that version of button code.
Since you're using WordPress, you might also be interested in the new WP E-junkie plugin:
http://wpejunkie.com/