The Pinterest button will only display if there is an image in the K2 item that JFBConnect can find. We search for the Main image or for the first image tag we can find in the article contents. Without an image, Pinterest will not let you use the button as there has to be an image to share on the page.
If there is an image, but the button still isn't working, please let us know a URL where we can see a K2 article with the issues. We'll gladly help investigate.
Thanks,
Alex