If it's any consolation, I find that threads are frequently remaining Unread, even when they have been.
This has been going on for ages - and believe me, if you think it's tough having it happen when you are "just" reading, you ought to try it when you've got 3 forums to moderate!
Fortunately, I've been given time off for good behaviour / had my sentence reduced, so I'm just moderating C&I for the moment (actually due to work commitments)!
The problem isn't so apparent when you've only time to look at the site once a day.
SD