facebook like over pop-up

We run pop-ups on one of our site once in a while. And I notice that the “facebook like” (only not other share buttons) shows over the pop-up. I tried z-index in CSS but so far the only affected was the facebook comment box and not the facebook like. Any ideas on how to fix this?

Thanks

http://wordpress.org/plugins/really-simple-facebook-twitter-share-buttons/