There could be several reasons for this happening.
Look at the creation date of the files if they look
like 1/1/80 or similar very early creation date it
could be a known issue where Windows and NetBackup
interprets these files as being created after the
backup time. Here is the technote that discusses that
issue: http://support.veritas.com/docs/252149.
Also, you could increase the logging on the client
that is affected and create the bpbkar log on the
client to get more information.

Jeff Redington

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"A wise man once said 'Wherever you go, there you are'"
      -Mike Brady