Vault - Duplication job is skipping some images

Hi

In my environment, Vault profiles are configured for Duplicating the backup images from my disk to tape.,

Everyday VAULT profile is configured to run using the respective Vault Type Backup policy.,

Everyday my vault job is getting failed with "306" error code and the respective Duplication jobs are getting partially successful with the below output.,

03/24/2015 22:59:03 - Error bpduplicate (pid=25238) Status = successfully duplicated 323 of 348 images.
03/24/2015 22:59:03 - Error bpduplicate (pid=25238) Status = the requested operation was partially successful.
03/24/2015 22:59:03 - end Duplicate; elapsed time 2:52:16
the requested operation was partially successful  (1)

When i search in the vault session logs., i can see the file named as "failed.images" file.,

16:00:18 INF - Duplicating policy xxxxxxxxxxxxxx schedule Full backup id xxxxxxxx copy 1 created on 03/13/2015 21:00:06 on source
 path @aaaac
16:00:19 INF - Waiting for mount of media id DR0113 on server xxxxxxxxxxx for writing.
16:00:22 INF - Beginning duplicate on server xxxxxxxxxxx of client xxxxxxxxxxx, reading file @aaaac.
16:01:05 INF - Beginning duplication on server xxxxxxxxxxxxx of client xxxxxxxxxxx, creating copy 2 on destination media id DR0113
16:01:24 INF - host xxxxxxxxxxxx backup id xxxxxxxxxx read failed, media manager killed by signal (82).
 
16:01:24 INF - host xxxxxxxxxx backupid xxxxxxxxxx write failed, media manager killed by signal (82).
 
16:01:24 INF - Duplicate of backupid xxxxxxxxxx failed, media manager killed by signal (82).

Am getting the above error in the vault session logs output., But when i manually duplicate the image its getting successful.,

Can you please advise us on why some of the backup images are getting skipped on day to day basis.,??Also we are different backup images skipping on day to day basis.,

NBU master server version is 7.5.0.6

Running in LINUX platform "Red Hat Enterprise Linux Server release 6.6 (Santiago)"