For step 3,  it is better to use nsrstage than nsrclone.

The nsrclone command with -b will require a clone pool.

With nsrstage, you can specify any pool. including the same pool the source tape was labeled with.

There is nothing to troubleshoot. This is the ehavior of each "sequential storage media".

And it is nothing specific with respect to NetWorker.

What you can do is that you consolidate the backups as follows:

  1. Determine the tapes you can recycle (you need at least 1 (re-)usable tape) und re-label it/them

  2. Set all other tapes ro read-only:   nsrmm -y -o readonly

  3. Start migrating the save sets one-by-one:  nsrclone -m -b -S

  4. Once the tape does not contain any valid save sets any more, re-label it

Of course you must have at least 2 tape drives to run a migration from one drive/media to another.

The most obvious reason for this effect is that you have media which contains save sets with different retention policies.

To avoid that you better use a separate pool for each retention period.