I think it's important to remember that sometimes people join the site just to have a look at whats going on.
Then for what ever reason, they may not access the site again. So this doesn't mean that sent pm's are being ignored, it just means that the recipient of the pm is no longer around.
If a pm that has been sent doesn't get a reply and you know that the message has been read, don't take it to heart that you don't get a reply, it's really not worth 'cuddling' your head about. Just my opinion