Changes to files on /nfs/gpfs may not be backed up during the following evening's backup, as would normally be expected. The backup software is attempting to recreate a full backup of the filesystem on tape, rather than nightly snapshots of changes, and the volume of data is so large that the operation is being split across several evenings worth of backups.
The problem is isolated to that filesystem, and does not impact home directories or other project space filesystems.
Systems staff are working on resolving the underlying problem with the backup routine.
UPDATE: We are currently "caught up", and backups are behaving normally, but it is still unclear what caused the original problem, so it could occur again.