Did you give the correct username/password for sqlplus in host.csh. This will be the username under which you compiled the procedure.
Make sure the background process is always runing by
typing the unix command ps -ef | grep unix_user.

This is what you should be able to see if everything works fine.
sqlplus > exec host( 'ls -l' );

The output of ls -l will apprear in the inix window.

Good luck !!