How to Fix Sketchup Thumbnails Not Showing 2015 / 2016 / 2017 / 2018 / 2019 / 2020 / 2021 Tutorial
The cause of your thumbnails not showing mostly because of you recently just installed newer, older, or different version of sketchup into your PC. So, to fix this you need to set your Sketchup manually as a default apps for the current version of Sketchup you use right now.
In other ways, you can just trying to install the latest version of sketchup. It'll automatically make the current sketchup version as apps by default. And the thumbnails should appear by now.