For some reason, I feel like there shouldn't really be a YA author or a YA book that I'm not familiar with. But that is far from the case. All the time, I'll run across some hit--something with three hundred Amazon reviews and a movie coming out--that I've never heard of.
Maybe it's because I sort of know about all of the science fiction and fantasy books. For instance, a book rarely hits an award list without me having at least heard of it. And often I hear about things when they come out. Authors, too, rarely come out of the blue for me in the SF world. Usually I'm aware of all the notable debuts. Or at least I feel like I am. That's obviously a mirage, since the SF world is very broad. For instance, things happen over in the military SF and in the big fat fantasy end of the pool that I'm often unaware of. But I can at least fool myself into thinking that I know all about it (even if I actually read very little SF/F nowadays).
Not so in the YA world. I know almost nothing. To some extent, I think this is because more YA novels are published each year than SF/F novels. The YA world is substantially bigger. It also spans four subgenres that don't necessarily have too much to do with each other. There's the straight-up YA romance, like Anna and the French Kiss, which is mostly about a guy and a girl getting together. And then there's the YA contemporary (what I write, nowadays), which is composed of realist novels of a variety and types and plots. Then there's dystopians (which are kind of on the outs, but still exist), like The Hunger Games. And YA fantasies. I know the least about those. But I know that the YA world has both urban fantasies / paranormal romance (i.e. Twilight) and straight-up secondary world fantasies.
Basically, I've read almost nothing in two of those categories (romance, fantasy*) and almost nothing in a third one (dystopia). I've read a number of contemporaries, but even there it's not a very impressive number (maybe 40-50 in the last five years). So it's no wonder that there's still plenty that I don't know about.
To a certain extent, non-contemporary YA seems to be shut out of the buzz machine, too. So while I might hear about a book like Leila Sales This Song Will Change Your Life, which (although it is kind of a breakout) didn't necessarily become a bestseller, I wouldn't necessarily hear about a YA romance that's sold hundreds of thousands of copies.
*Basically the only YA fantasy I've read is the Flora Segunda series, which was amazing.