My thumbnails are not appearing like before. Why?

It is a known issue with SketchUp 2021.

Hi marceltanure,
You might try the “Component Finder” utility offered by Flextools … it offers thumbnail views of all SketchUp files (not limited to Components), and you can enlarge the thumbnail views up to several inches wide! Although the Flextool full suite is a paid extension, I believe they offer Component Finder for free?
Good Luck …