file sharing

apparently when opening a (Excel) file that is already opened by another user, we are no longer getting the “this file is opened by …open as read only, notify or cancel” message.
Users open the file normaly, edit, save, no errors, but if the file was opened previously by someone else, nothing is actually saved.
I guess the file is opening by default as “read only” without notifying the user.
We recently switched from Novell to Win2k file server if I create a new folder with a new file, that new file will come up with the message….
where do I have to look ?