Can't see the user name of the person who has the worksheet locked

Hello,

I'm using Excel 2007 on a network and we're having a problem
identifying users who have worksheets open. Anytime a user opens up an
older excel file with the *.xls extension we can see the username of
the person who has the file open. However, if they open up a file with
the *.xlsx extension (the 2007 extension) we can't tell who has it
open, the message from Excel is that "another user" has the file open,
but it does not say who that user is. Does anyone know why this is
happening and how to fix it?

thanks