Hello,
Make sure , no one at your site uses Tape library functions to move, add 
or eject tapes, Networker can get confused when work is performed outside 
it's knowledge, then double check your Lib log and make sure nothing odd 
or errors which have not been corrected on the Tape Library.
Also make sure there have been no drive re-sequencing problems.
I have seen this problems occasionally, even with everything being right, 
You may have to remove the volume from media dB and re-lable.
Reseting lib\Networker via nsrjb -HEvv  should help. If it's an repeating 
issue, you may need to work with EMC on it.
HTH

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