Button not working on square images

Hi,
I’m loving this plugin but I’ve just noticed it’s not working on any of the square images on our site. For example in this post and this post the button does not appear on the square images at the beginning of the post but does appear on the rectangular images at the end of the post.
Any suggestions?
Thanks.

http://wordpress.org/plugins/pinterest-pin-it-button-for-images/