Newsgroup shows new message(s) but...

[MozillaMonkey]

The thread is underlined, which normally indicates a new message(s).
But when I open the thread I see no new messages!

What can I do about this?
What is causing this?
TIA

[MozillaMonkey]

Oh dear me.
Is there no way to 'reset' the non-cooperating Newsgroup thread?