Why doesn’t the Pinterest button show?

The Pinterest button assumes you want to share an image that represents the whole content of your webpage / article. As such, NGFB will look for a custom Image ID in the Social Settings metabox, a featured image, and lastly, an image attached to the post / page (see Working With Image Attachments). The Pinterest button will create and share an image based on the Image Dimensions chosen in the Pinterest button settings. Make sure your original / full size images are larger than the Image Dimensions you’ve chosen.