To narrow it down to a conflicting theme, just switch to the default WP theme (2011) and see if that does it. If it's not that then disable all other plugins and add back one by one until it breaks.
Other than that it's hard to say. I"m not sure how technical you want to get, but it looks like an iframe is NOT getting generated. It should be (for now). That's how the original Pinterest button (and many other sharing buttons) are designed.
I am working on an update to the plugin which allows you to use a non-iframe button. Not sure on a release date yet though.