The thing with PMs, is often people word them in a means like they are simply making a statement to you and not a question, and thus don't need a reply to them. I am guilty of this too as I send messages hoping for a reply, and don't get it and then re-read what I sent and feel it could be interpreted as just a statement to that person.
The other thing is, if you reply, they then send one back, and you reply, one back. And it gets confusing who should get the last word and just let the conversation die its natural death as if everyone just sends a quick response. The other person then may bee obliged to do the same.
Another thing with PMs is sometimes people send them late at night when I am just in browsing mode on the forum and not in posting mode - as in I cannot be bothered to sit up, grab my keyboard and start typing. And its nothing personal. Its just I am too tired or exhausted to type.
I do think if its a case of someone is ignoring you because they don't like you. Then they should stop being so immature and just say in a brief line they don't wish to chat with you. (I personally would never ignore someone because I didn't like them by the way)