I can’t seem to find any rhyme or reason to it. Sometimes the casting button just doesn’t appear as an option. I first noticed when I tried to watch something that was a youtube video that’s in 4:3 so my best guess was maybe that somehow had something to do with it but I tried various videos at different aspect ratios, frame rates, resolutions even different videos on the same youtube channel as where I has actually succesfully found the option presented.
This is probably happening for all sorts of video but I noticed it with youtube and did my testing there because that’s where this extension works the most reliably and where I can be confident some other weird factors about how the website is setup aren’t responsible for a video not being able to cast.
Have you checked when the last time the extension was updated?
YouTube has gone through a lot of back end changes to try and force people to watch more advertising than videos so it could be that YT has broken the system the extension was using.
It says last updated in December 23. I can’t seem to find any way to manually update, on the github page there doesn’t seem to be a stable later release available. I hardly ever use the extension so it was kind of disappointing when on a whim I wanted to put up a trailer for a movie on screen and found it suddenly didn’t work. Oh well, it’s a complicated thing to pull off I imagine.
It’s likely an unfortunate casualty of Youtube’s enshitification