Every time I look at questions, whether on Stack Overflow or other sites, I find myself not being able to answer most questions. Thus, I ask more than answer, which is what I hate.
I think that I have enough knowledge to be able to answer a wide variety of questions.
Is it normal to be hardly able to answer questions, or do I need more knowledge? Is it because most questions are being answered, so that questions are being specific to some technologies or libraries?
It's normal.
First, Stack Overflow is enormous. It boggles my mind to think about the question rate there. I'm active on sites with lower traffic, fewer questions, less users, and, in general, less activity. And, of course, I can't come close to answering a meaningful percentage of them (well that statement isn't true on all of the sites).
Second, nobody is expected to know everything. Jon Skeet doesn't know everything (I think), and he's possibly the most famous person on Stack Overflow (besides Jeff Atwood and Joel Spolsky, of course). You're a normal human being.
Some folks are experts in a few small subjects, and so they spend time answering only those questions. That's fine.
One of the biggest misconceptions new users have is that those who provide answers are able to do so easily, and without much work.
The truth is, answerers are engaging in a learning process alongside question askers. Stack Exchange and Stack Overflow are all about documenting specialized knowledge, and nobody is born knowing everything.
To quote HDE 226868:
Stack Overflow is enormous. It boggles my mind to think about the question rate there.
The folks who answer questions have just opted to get really knowledgeable in a few well-defined areas. A few select tags on SO. Or perhaps a subject like Raspberry Pi, or Judaism. Then, they hang out in those places where those questions get asked. And they watch, until they find a problem they think they can tackle.
That doesn't mean they already have the answer in their brains, just ready to go. No, when someone sees a question they think they can answer, they're usually making a decision to use some of their valuable time and energy to figure it out. Since they're already knowledgeable about this thing, they just happen to have a head start on the OP in terms of knowing where to look, whether that means digging up the right research papers or reproducing a bug and knowing why a given error message is being thrown. Answering is its own R&D process.
If you're having a hard time finding questions to answer, I recommend doing two things:
1) Narrow your focus. Pick just a couple technologies, libraries, or operating systems to experiment with and read about on and off of Stack Exchange/Stack Overflow. Hang out on the corresponding tags.
2) Watch for questions which pertain to your focus. Expect that you will see questions which deal with familiar-looking problems sooner or later. When you do, be prepared for them to take research and effort to answer.
Like HDE 226868 said, not coming across anything you can answer is entirely normal. That said, if you decide to put your personal effort into a few select and concentrated areas, it's within your power to change that.
:-)