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.