Why Won't That Silicon Valley HeadHunter Talk to Me?

Maybe it's because you are not in the niche or specialty that this particular recruiter works in. Many times, headhunters are quite specialized - they only work a specific niche within semiconductors, for instance, and only recruit design engineers with master's degrees in electrical engineering for that work. Further detailing the specifications they're looking for, the recruiter may only be interested in people with experience in a specific product type, like systems on a chip. If you don't meet every one of these specifications, you are not in their target space.

And they won't call you back, or even respond to your email. Which is bad manners, but understandable... The better headhunter will give you feedback, tell you what you're missing that is on their specification. And in a perfect world, suggest what other experience that you need and how/where to get it.