This is probably a permissions problem on the directory in which the
backend is installed.   The users must have create/delete privileges
to that directory.  What is happening is that Access can't create the
.ldb file which allows multiple users to update the MDB.  So Access
only allows one user at a time.

One simple way of testing this is to ensure the users can create and
delete a file in the network share.   Any file, even using notepad, is
enough to test this. I simply don't trust all those permissions screen
within the OS.  You never quite know whats lurking behind the advanced
button.  So test this yourself.

Sometimes it could only be one user who does not have create
privileges to cause problems who just happens to be the first user
into the MDB at that moment in time.  Then all the other Access users
can't access the file because the first user is in exclusively.

"I found that I could keep the permissions set to Change, but had to
ensure that the directory in which the db resides was set to not
inherit permissions from it's parent.  It seemed that every time a new
user logged onto a given machine, it got messed up."

See ACC: Determining Which User Has Opened Database Exclusively
(Q169648)
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q169648
Essentially you must use tools on the server to determine who has
locked the file.

Tony