If I recall correctly the FTM cookie that keeps track of what posts you have read explores after thirty days. So if you are like many of us that never log out that cookie will expire after thirty days and you have to log back in to reset it. If you actually Log in during the thirty day period the cookie's expiration date is reset. Any thing that deletes/sweeps that cookie has the same effect.
I have also experienced what Ryck described. I often see this happen in one tab, when I have FTM open in two tabs/windows, but the other tab is still okay. The only explanation rationalization I have for these phenomena are there are two sessions using the same cookie and when one session tries to write to the cookie and is blocked by the other session it simply disconnects.