Two Users Acessign Same FIle over LAN…..Help

I have two sometimes more, users accessing an Excel spreadsheet on a network drive. The dialog box that used to come up that says someone else is working on the file no longer pops up. It used to give you the choice of Notifying the user, Opening a Read only copy or cancelling the request. How do I get this dialog box to come up. I need this because there have been several times when two users are working on the file and they overwrite the other persons changes. Please let me know. Thank You.