dixie, I wonder...
There are various ways in which message stop being "unread posts since last visit" ... when you experience this, do you ever click the end of "No unread topics found since your last visit. Click here to try all unread topics." to see if there are any unread messages at all showing?
If you load the site, do nothing then come back some time later ... it would only be messages posted between those two events which would show in your list, so at a quiet period, there may well be nothing showing. But all messages you haven't read would still be available through the link I mentioned above.