One thing I've seen lately is that YouTube will recommend a video that I've already watched all of it. And they know I've watched it because I leave the watch history on and it has the red bar showing how far through the video i watch.

I also don't know if it's because of other users behaviors or maybe channels pay for the exposure, but I've seen certain channels or topics take over the recommendation list more than other topics. Or there's the issue where the recommendation list beside a video is all "recommended for you" and not even related to the video. On mobile they had to add a button for showing videos actually related to the video you're watching, but it feels like sometimes even that's hit or miss.

I'm okay with a recommendation engine when it doesnt get stuck in this loop of recommending the same videos over and over. For now, I just delete my YouTube history about once a month to get new recommended videos.

Part of this may be youtubes fight to subvert people leaving keywords in the descriptions of videos so instead they're analyzing different things trying to build the heuristic. I know there's a lot that goes into the engine so it could be any number of things.