Hello. This message is a bit ambigious - can NW not backup a certain volume or can't he recover from a certain volume?
If during recovery, it might be due to a defect media.
If during saves, why don't you run at the client
save -v -s server your_volume
This should give you some more details.
You can of course also run savegrp for this client to gather more info.
Please also look in the manpages for savegrp and to the
Error Message Guide for possible solutions.