Solution 1 - Clear Browser Caches & Cookies
Believe it or not, just like restart your computer, clearing the browser caches and cookies is regarded as a priority and somehow works to fix YouTube preview not showing on Facebook. Although caches & cookies are convenient for a quick access to visit website and load massive information, they also give rise to a load of inexplicable issues. You can always find "Clear Cookies and Caches" on "Settings" window of each common browser.
Solution 2 - Update Browser & Disable All Extensions
Solution 3 - Make Sure YouTube Video is Public & Processed
Solution 4 - Force Facebook to Update YouTube Video Info
You know what, Facebook will analyze, load and record the data from YouTube video link once it's shared for the first time. That is to say, even if you've updated your video information on YouTube like changing thumbnail, it probably doesn't synchronize with the subsequent shared video on Facebook, because Facebook has added the first-shared video info the cache. That's the common reason why YouTube link no thumbnail on Facebook or with an outdated information. But don't be fret, this problem can be easily fixed using Facebook's Sharing Debugger tool.
Step 1. Log into your Facebook account.
Step 2. Open Sharing Debugger page in another tab: //developers.facebook.com/tools/debug/.
Step 3. Copy and paste the URL of YouTube video that doesn't show thumbnail on Facebook in the Debugger link box. Then click "Debug" button.
Step 4. This tool will force Facebook to refresh the cached information and update the latest YouTube video metadata, including thumbnail, title and description. Press "Scrape Again" button if there is still no thumbnail or old information.
With the 4 solution, YouTube thumbnail not showing on Facebook problem is supposed to be solved easily. On the other side, if you consider it as a hidden trouble, you're advised to download YouTube video and directly post video on Facebook. If this post is of some help to you, please share it through your social media. Cheers!