There are reports of uBlock Origin occasionally not working on YT, and I experienced this 2 days ago on one video. I can't recall which video was, I thought I clicked on the wrong video, then realized it was an advertising, checked if uBO was active and indeed it was. Happened only once to me, others were apparently less lucky. This could be the result of different tests performed to discover adblockers limitations.

For the last few weeks uBlock on my devices seems to have just stopped Youtube ever playing anything. I've read plenty of threads about it but gave up and turned it off for the domain.