Just a theory, but—

I find that if I access a thread by clicking on the thread title, it changes color from blue (link) to black. But when I come back to it, if it's blue again, I know a new message has been added.

However, if it's a long thread (over 50) and I access it by clicking on the right hand column (number of messages) and go to the latest page, the thread title doesn't change color.

Could this be it?

Don Firth