My experience is similar:
From time to time I partake in the HN freelancer thread. Not that I ever got any business out of it. I still think in principle it would be nice to do business with fellow HNers, that's why I keep trying it.
Usually, I get about 2-3 inquiries per month from the Freelancer thread - by far most of these from people who apparently came through a search engine and aren't HN users at all. Of these, 2 are general carpet bombing mails, asking me for things that I didn't advertise and can't/won't provide ("would you like to move to Berlin and work as an entry-level tech support person?"). Sometimes, that leaves one real contact.
Here's where things start to get on my nerves: there will typically be an exchange of 3 to 4 emails back and forth. It starts with a project introduction (which is often excellently detailed), then we're moving on to references and code samples, then we're talking fees (even though I explicitly state them on my profile website), after which it's all "I'll totally do this project with you", and after that: nothing. In two cases, there were even 30+ minutes Skype conversations, all for nothing of course.
Last month I decided to do something different, be more proactive: I sent out emails to people looking for freelancers on HN. Same procedure (see above), same results. But I got something very interesting out of it: a few of them told me they received upwards of 30 applicants! Holy crap, even for the lamer job postings!
It's now clear to me why the freelancer thread doesn't work, at least for "normal" HNers like me. As a freelancer, there is so much competition, the chances of success are exceedingly slim. Companies are drowning in applicants. Of course they're going to choose either the cheapest one, or if you're lucky enough to inhabit something of a niche, the most famous one.
So why are they not responding? Why are they just breaking off communication after a short exchange? Because they don't have any reason to value our time. There are just so many of us.