Hello,
Make sure that you use compatible Block size if you are in multi OS 
environment, usually, smallest compatible.
also, please check your Tape drivers, SAN config, if the Tapes are getting 
into error situation ( also check CDI) even if they are DDS and lets say 
one of the shared drive is having issue, it would effect other instances 
of it.
I believe when Networker encounters errors on the drive\tape it may mark 
the tape FULL prematurely.
The one thing to do is isolate which might be your problem client to it's 
own group and rerun backup to see if  the problem is only isolated to 
couple of client or is it system wide.
HTH

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