The script loads when navigating directly to a video page, however navigating to a video from a non-video YouTube page (i.e. one whose pathname does not start with "video") does not load the script, since YouTube is a single-page application that messes around with history instead of (re)loading. For a concrete example, try visiting the main page first and clicking on any video - the script will not load, until you force a page reload. I suggest removing "video" from the pathname in the @match pattern.
The script loads when navigating directly to a video page, however navigating to a video from a non-video YouTube page (i.e. one whose pathname does not start with "video") does not load the script, since YouTube is a single-page application that messes around with history instead of (re)loading. For a concrete example, try visiting the main page first and clicking on any video - the script will not load, until you force a page reload. I suggest removing "video" from the pathname in the @match pattern.