Changes to meeting cannot be saved - error message

I am a delegate on my boss' calendar. Lately, when I try to update a meeting
and resend it, I occasionally get a message that says that "changes to the
meeting could not be saved. The meeting has been updated by another person.
Close the meeting, then re-open it to make your changes." I do that, and it
still won't work. I cannot resend the meeting with the updated data. Any
ideas of why this is happening, and what I can do about it? We are on
Outlook 2007. Thanks!