Find based on CreationTime property fails

[vb user]

Hello,

I have an addin which does a search of the Inbox for unread messages
that factors in the MailItem.CreationTime property.

On Exchange only, it appears that if Outlook is running overnight,
that
the next morning the Find stops working. The Outlook object model
simply
returns no items.

When the Outlook addin is in this state, a search for unread messages
which does NOT use the CreationTime property works just fine.

Any suggestions?

thanks