When you share a post on Facebook, or when Jetpack Publicize publishes a post to your Facebook page, Facebook crawls the page and looks for Open Graph meta tags in the head to build a complete post preview (with an image, title, description, ...). Jetpack automatically adds these open graph meta tags for you. I checked a few of your latest posts, and the tags are there.
Unfortunately, and as you mentioned, Facebook is not able to access your site to read the meta tags. For some reason it seems to be blocked before it can reach your site.
This can be caused by different issues; you can follow the instructions here to try to solve the issue: