YouTube is weird, man...
It's personalized for everyone to SUCH a degree, it's really not fun anymore. I watched a lot of movie-making and analysis videos (like "Every frame a painting"). But now my entire Youtube-page is bombarded with these types of videos and less else... I also like "Crash Course" or DIY-videos. But I watch them fewer, and now I have to seek them out specifically, because I don't get recommendations for these anymore, because they're overwhelmed from the one other category of videos...
Also, there is almost no useful discussion about "YouTube" in general possible anymore - it's SO personalized, no two people have a similar experience on it, not even with close friends.
YouTube really should split up it's user-interface into "this is generally popular", "this is the main type of videos recommended for you" and "hey, here is some stuff related to the stuff you checked out infrequently might new but interesting for you!"
Because Jesus Christ, I don't want to always be bombarded with the familiar stuff in less and less good versions, I really would appreciate to randomly stumble upon some new good stuff I'm unfamiliar with!
Goddamn.
Just clear your browser cache and start hitting more of the YouTube videos you like, and you'll see those type of videos re-populate on your 'YouTube recommends' lists.
And again, use the "not interested" option on individual vids and channels. It's not perfect, but it does make a difference.
You can also clear your viewed & search lists as a sort of reset.