Is there a typo in the client name when you entered it in the Backup, Archive & Restore (BAR) GUI? (e.g.cleint_nmae instead of client_name) Must use the same name as it appears in the backup policy(ies). This is mainly true if you use the Windows GUI as the Java GUI populates the list of clients automagically. (certainly in NB6.5.4)
Have you chosen the correct source & destination clients?
Can you connect to the client 'normally' through the Host Properties/Client tab in the Admin Console.
Maybe a bit more info would help us tie this down for you: NetBackup versions, operating systems, type of backup you are restoring & procedures followed etc etc. You are obviously doing this right in that you say other clients are working ok, so we need something to differentiate whats not working from what is.