Mysteriously, it would work properly if I just gave facebook enough time (usually a day or two). So that made me think it was a browser caching issue, but nope. It was something from the facebook side of things.
I discovered Facebook’s Debugger Tool on the developers side of the site.
I pasted the link that wasn’t loading properly and clicked debug to see what was wrong.
I couldn’t figure out how to fix what it recommended, so I didn’t do anything.
I decided to try one more time to see if the link would load properly and LO AND BEHOLD, it loaded properly this time.
I’ve been using this trick for the past six months on the off-chance that the link doesn’t load properly. (Maybe one once every 20-30 posts.)
So I don’t know what’s going on when I paste the link into the debug tool, but when I do and go back to post the link on my timeline, everything loads properly (including the thumbprint, title and description). I don’t have to wait a day anymore. It just makes things work.