I have had only a few PM's but as far as I could tell with each one as soon as I logged into the site a little box popped up telling me I has a private message. That makes it very easy to notice I have a PM. As far as answering, I couldn't' imagine myself ignoring it and not responding. I would feel like I was being rude not answering. However I could easily see see where if you got busy and didn't log into the site for a long time it could get missed.