I was always under the (apparently mistaken) impression witnesses could be silenced/censured/sanctioned -- even if they're seated congressmen or senators themselves -- for refusing to answer questions or evading them like this. At the very least, can they not be interrupted and dismissed so as not to waste time at the hearing, and held in contempt to prepare themselves for a second try at behaving themselves and actually answering the fucking questions they're asked?
I ask this seriously. These aren't courtroom proceedings. These are post-high-school debate club debates, with actual rules and requirements, with the power of congress (and "contempt of congress" therein) behind it. If you're going to sit there dumbfounded for almost half a minute either literally silent as you scramble for an answer or you're just going to repeat yourself over and over as you stall for time because you can't answer a question, surely that can be punished at this level of government, shouldn't it?
She is literally elected and paid to be there as an expert witness to answer such questions. There should be a penalty for such blatant failures.