shared workbook issues

I am very confused on the issues of a shared workbook. I have a workbook
that is used by multiple users in an office enviroment. The users have full
access to the network drive where the shared workbook is stored. The
workbook is being shared out correctly from the tools-shared workbook menu
(users can modify the workbook, etc.) but one user that opens the workbook
cannot save the file to the shared folder and when she tries to save the file
to the shared drive, no error message pops up and she is "kicked" out of
excel. I can check the windows log file to see if any error messages are
there but excel itself does not show any errors.

But the funny thing is that when I connet to her machine remotely and she
opens and saves the workbook, the changes are saved successfully with no
error or her being kicked out.

Can someone tell me why a user would be "kicked" out of excel without an
error message.