NW 6.5sp3

I recently migrated my 5.1sp8 network to a new box running 6.5sp3.
Since that time, when USER B opens a file that USER A has already been
opened, USER B does not receive the warning that the file in use by
USER A and will be opened as a Read-Only file. It is only when USER B
goes to save this file that that it gives them a message about saving
the file in a temporary fashion.

How can I get the Read-Only message to appear when a second user opens
an already-opened file?