In Word 2007, when I save a document with Tracked Changes, Word looses the identities of the individual Reviewers and indicates that all Tracked changes were made by "Author".
Probably the issue is due to some security settings.
Click the Office button and select Word Options | Trust Center | Trust Center Settings | Privacy Options. Look for Document-specific settings group and uncheck Remove personal information from file properties on save
If the option is grayed out hover the mouse over the option to see an explanation for this reason.
If the document was created in Word 2003 and saved Remove personal information from file properties on save option checked, you should be able to uncheck it in Word 2007, but it won't bring back the author information that was already lost, but any later tracked changes will be saved.
For Office-2003 users:
Go to Tools | Options | Security and uncheck Remove personal information from file properties on save