'View Posts Since Last Visit'

Hello! I'm hoping someone can give me some pointers on this problem I'm having. Started up a forum a few weeks ago, it's going really well, managed to get pretty much everything sorted out between myself and my co-owner, however we've noticed that there are times when we use the 'View Posts Since Last Visit' option and there aren't any... even when there should be.

At first I thought it had something to do with the new day starting, but I've been online when it's 11PM Forumotion Time and the posts are still not showing up as they should be. Any pointers would be greatly appreciated! If you need more details, please ask because I'm not sure that I've explained it well, not being of sound technical mind!