Why do potential supervisors not reply to my emails?
I don't know if this applies to you, but it might help: I receive quite a few e-mails from prospective graduate students. I am looking for new students, but I delete most of the e-mails that I receive without reply. My reason is that most of the e-mails are obviously form letters that are sent to multiple professors with no indication that the student knows or cares about my own research interests. If they didn't spend the time to assure that I might be a personal match with their interests, then why should I waste my time replying?

I don't know if this is your case, but I do recommend that you spend the time (up to 30 minutes per professor) to get to know their research interests based on their websites and their publications on Google Scholar. Only e-mail people whose topics seem similar to yours. (This might eliminate several professors.) When you do e-mail, one paragraph is all you need: you just need to explain directly to the point how the professor's research interest match your own.

Of course, I can't guarantee that anyone will reply you if you do this, but when students do, I always reply them and enter into a serious e-mail exchange. Actually, even if the student's interest doesn't actually match mine (or they pointed out an old research area that I no longer pursue), as long as I see that they made a sincere effort to know my research (again it takes up to 30 minutes per professor), I always at least give them the courtesy of a reply (though I can't guarantee that anyone else might).