You should check in the UNSDaemon.ini file. There is a [Purge] section that contains the parameters used by Notification service to purge the .nob files.
These are the default settings:
; Use PurgeSelect to enable purge of checkpoint files.
; Options for this parameter are:
; None - Disk purge is not enabled
; All - Both completed and incomplete notifications may be
; CompletedOnly - Only completed notifications may be purged
; SetPurgeCompleted hours to indicate how old a completed notification is
before it is eligible for purging
; SetPurgeIncomplete hours to indicate how old an incomplete notification
With PurgeSelect=CompletedOnly only the completed notification will be purged, to make also the incomplete notifications to be eligible to be purged, you need to set PurgeSelect=All and then recycle the notification service.