Does the account that clients are using have fmnet priviledges? (security setting)
FileMaker networking goes via port 5003 - ist this port open? (firewall)
Did you enable sharing for each file? In the same dialog where you turn sharing on, you select each file (they must be open) and select "all users".
Note that Phil asked was sharing on for 'all files' (on the server), not 'all computers', as you originally posted. There is no need for the client computers to have their sharing turned on in order to access a file on another server.