I had the same issue and solved it. I hope this may help others.
The issue is related to the operative system not being able to extract existing data.
In my case, Windows 10 simply refused displaying individiual thumbnails for Sketchup 2021 files. I had hundreds of Sketchup files whose thumbnails showed only the generic blue Sketchup logotype. All other Sketchup year versions’ file thumbnails worked well, even if saved from Sketchup 2021. Up till now, I had never seen a Sketchup 2021 file thumbnail. I tried repairing my Sketchup 2021 installation, to no avail.
Not seeing individualized Sketchup file thumbnails brings the realisation that thumbnails are indispensable as an aid for quickly finding the right Sketchup file among thousands, no matter how well organized and named they are.
In my case, I solved the issue by:
• opening Windows’ Registry editor;
• selecting the root (Computer) and then searching for “ThumbsUp.dll” (this is Sketchup’s helper library that enables thumbnails);
• modifying a few existing registry entries’ value data (I do not remember exactly which entries), replacing their link to an older version of ThumbsUp.dll for a link to the most current version instead – in my case located at “C:\Program Files\SketchUp\SketchUp 2021\ThumbsUp.dll”;
• restarting the computer.
After that, Windows 10 immediately recognized and started displaying Sketchup 2021 file thumbnails. The thumbnail data is embedded. There is no need to resave any Sketchup file.
– Bear in mind that tinkering with the Windows registry is frought with the possibility of terminal mishap.