Part of the problem may be that some of the recovery points were manually deleted rather than doing it through Ghost. This likely threw Ghost off. Going forward, delete recovery points through Ghost itself. To reset this, you'll likely need to create a new backup job, get a successful backup, and delete the old job. Use Ghost to delete the recovery points from the old job, even if they do not exist anymore, by going to Options>Manage Backup Destination.
One thing to consider is rather than using the Optimize feature, simple set Ghost to retain a certain amount of recovery point sets. For example, if it's set to retain 2, you will always have 2 complete backups. If room is running out, you will be warned so you don't miss a backup.
-Andy
