If the thumbnail of your digital publication is not pulling through when sharing your content to Facebook, this is most likely due to the delay in the Facebook API from fetching the information. However, there is a way that we can push through the thumbnail to facebook so that it appears immediately.
Clicking on the image below will take you to the Facebook Object Debugger:
Fear Not! This sounds more complex than it is, as all you have to do is punch the live link of your publication into the Input URL field and hit Fetch new scrape information.
Once the scrape is complete, the information will show under the When Shared, this is what will be included section along with a preview image to show the thumbnail and digital publication title. Once you have confirmed that it has pulled through the thumbnail, you can now share this link through Facebook.