Hillary has a problem telling the truth. It's just not something she does. There are a lot of reasons for this, but one of the biggest is that her whole career is predicated on 30 years of lying. Her life story is essentially one gigantic, tangled, web of falsehoods, half-truths, spin, and obfuscation.
The thing is, when you've lived that way for so long, you can't just stop. In order to push forward, you have to keep lying, and every new answer has to square with the previous set of fabrications. That's a lot to keep straight, so it becomes much easier to simply avoid questions altogether.
...And that's what Hillary's been doing.