Lately, when I share my posts on Facebook, the image / thumbnail does not show up. This used to work. And it is only true for this wordpress blog (I have a Blogger blog that still shows the images on FB).
As @timethief mentioned, if you experience issues with some of your posts, it might be worth entering the post URL in the Facebook Debugger. It will clear Facebook’s cache for that page, and force Facebook to crawl the page once again to look for a title, a description, and images.