I’ve just found out something interesting that I haven’t seen a fix for elsewhere.
Using this plugin for WordPress (thanks @alexmoss), a client reported a problem that the plugin was saying ‘URL Unreachable’, as per the screenshot below.
I followed all the advice here but found that the settings were all correct.
This page advised using the Facebook URL Linter. I found no problems with the URL but noticed the client had used an ampersand character ‘&’ in their title.
I checked blog posts without that character in the title and hey presto! That was the common feature.
So, the fix for now is to avoid ‘&’ and possibly other special characters in your blog post titles. Looks like something Facebook needs to fix.