There can be a number of reasons for the above:

-Faulty bar codes on the media
-Slot mislalignment meaning the bar code reader is unable to read the bar code
-Old bar codes with slightly peeling corners etc

What I would suggest is taking one media that has been registered correctly by 
Networker and moving that to one of the slots not reading a bar code - if  you 
can then see the new media it is more likely the tapes bar code at fault - if 
it doesn't read the new nedia it could be the physical slot location at fault.

Are you seeing any errors on the library itself?

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