this has been an ongoing problem since the site opened
I think it is a blip with the people who supply
the forum, it usually sorts itself out

A couple of users have indicated that occasionally private messages are not popping up in a seperate window, or being noticed by the recipients.

I have checked all the settings in Admin and can find no problem.
Each user can define what happens on receipt of a private message by ticking the appropriate boxes in their profile.

This can be set to email you upon receipt of a PM.

Check your settings under...
Profile
Then preferences (along the top under the google ad)

Thanks
Steve