Hello Frank,
Assuming you have setup a retention rule for your Backup Management groups of max 28 days and you see snapshots which are older than 28 days,
In general what tends to break SnapManager applications is the volume option 'autodelete' which deletes the snapshots in ontap without notifying SnapManager about that, which in turn will cause the whole SnapManager backup set to be incomplete. When SMSQL then looks for older snapshots to delete and cannot find one of the snapshots in the backup set, it will exit in order to preserve the potentially needed remainings of the backup set. (think about accidental deletions when in fact you need either the database snapshot or the log snapshot).
So, what I would do is check that option and disable it in case is enabled (if you are worried about space, enable autogrow rather than autodelete). Autogrow is safer but you need more space in the aggregate.
a clean up in SMSQL is needed by using the delete backup wizard as shown in the attachment.
if this does not help,
I would log a case with support.
Domenico Di Mauro