I'm using the latest version 1.8.1 of the Video Thumbnails plugin. It worked for the longest time and then randomly sometime in the past few months (I think after a WordPress update) it stopped working and I have been uploading the thumbnails manually. I can't tell you the exact time or which update it was because I wasn't really paying attention to the thumbnails when it stopped working.
I deleted and re-installed the plugin and then ran a "scan past posts" with no luck. I am a novice wordpress user and know very little about wordpress and coding, all I know is the plugin recently stopped working.
My website: TeraBrite.tv
if you scroll down and go to the second page you will see where I neglected to manually upload featured images and the plugin has failed to work.
Also, in this category: http://terabrite.tv/vlerabrite-terabrite-daily-vlogs/