If you noticed that some of your Facebook shares (via our website or others) aren't showing the thumbnail preview prior to sharing, then don't fret. You're not alone.
We see it too and unfortunately there isn't anything we can do about that. Here is a summary of why.
According to Facebook's own documentation this is a known problem. The preview image isn't displayed until the Facebook crawler sees it, so the first person to share a page may not see a preview.
Their official advice is to use the OpenGraph Debugger and page by individual page process each URL you want Facebook to pre-cache. There are no other methods of having Facebook pre-cache images.
So with that sadly stated, the solutions here are limited.
If you don't want to use the Debugger tool linked above, here is our suggestion: