Bring back the WATCHED overlay to the videos you have already watched on youtube.
< Feedback on Return WATCHED badge on Youtube (with custom text)
You haven't installed it properly, or you are using something else that changes the thumbnails.
The style relies on the red bar on the bottom of the video.
There is an option to hide it if you want, just make sure you aren't touching the red bar with another script or style.
I'm using this all the time and everything is fine.
does not work