Video Thumbnails are gone and replaced by Icon

My Video Thumbnails in Windows (File) Explorer have gone and been replaced by the generic Icon for the program I select to play the video files (VLC, GOM etc). It's happened to my AVI and MPG files on ALL my Windows 8 PC's. The Folder Options are set to "Display file icon on thumbnails" and the Performance Options/Visual Effects are set to "Show thumbnails instead of icons". I tried using...