What version of Joomla are you using? Are there any errors on the page if you turn error reporting in the Joomla configuration area to Development? There's a known error in JFBConnect for Joomla 3.8 that may cause this (and I'm currently testing a bug fix release to go out within the next couple of days).
1) do you have the Open Graph - Content plugin still enabled
2) are there any og:image tags set on the page
3) can you provide a specific URL with the problem so that I can look at the tags that are generated
4) Is the problem still present if you rescrape the page on Facebook's Sharing Debugger?