I recently started having a problem with my thumbnails not showing on Facebook whenever I copy a URL and enter it onto my Facebook page. I had this problem before, and the solution was that I had to "Allow Pingbacks" on quick edit.
For the past few months, all of my posts have been working fine. I don't remember changing anything and it just stopped using thumbnail.
An example of a post that isn't working is: Example Post
If you need any additionaly info, just ask.
Thanks for your time!
BTW, I read other posts related to this topic and none solved my issue.