Read Only permission to folder where database resides

We have an Access database stored in a network folder, we then have a number
of users with full permissions to this folder and some users with read-only
permissions. At present a user with read-only permissions only seems to be
able to open the database when no-one else has it open. If they do manage to
open it (i.e. no-one else in it at time) then it locks the file and non of
the other users can access it. If the file is already in use when a
read-only user tries to open it they get an Access error message "Can not
Lock File" Anyone any ideas. Weve tried the database using Access 2000 and
Access 2003.

TIA
Ally.