The default installation of elm in RH6.1 only supports flock for file locking. flock of course does not work over NFS. If your mail spool is NFS mounted from another machine you need to use either the dot style locking or fcntl style. Thanks
Since Red Hat 5.2, all mail user agents use only fcntl locking. fcntl locking works if you start rpc.lockd correctly. Otherwise please report as a bug against knfsd.