Prospective PhD contacts potential supervisor but receives no answer after 2-3 emails, what to do?
To try to make a clear point: there is a range of opinions about how to respond to "cold calls" from people I don't know, etc. I myself find it easy to make a one-or-two-sentence response politely thanking an inquirer for their interest, but (if so...) that I "have no open positions in my group at this time" or whatever is suitable. For me, although I get a good number of these of various sorts, it takes less than 5 minutes a day, and I take the viewpoint that it is a good professional-social gesture to make that quick response (perhaps more worthwhile than yelling over the phone at phone-solicitors on old no-caller-id phones).

At the same time, at the other end, I know many people who take the viewpoint that sending them email no more obligates them to a response than do credit-card offers and other advertisements. And, indeed, given the ease with which we can mass-email, even with customizations, this is a fair, not unethical, not harsh reaction. Perhaps if my in-mails of this sort reached 10+ per day, I'd give up "trying to be polite", but my own current scale of "spam on behalf of earnest beginner in the business" is pretty low, so I can easily afford to be (superficially) "gracious" enough to respond.

In case people don't reply, I'm afraid you can't hold it against them, even while we admit that it would be great if they did respond, ... because there just isn't a general social principle that demands a response to all possible inquiries one may receive. Yes, it'd be nice, but it is simply not required.