You have your backup set retaining specifications set for 1 set. A backup set = Full backup (synthetic or not) + incrementals/differentials / The next backup set = Next Scheduled Full Backup (synthetic or not) + incrementals/differentials, etc... When the full backup fails, all of your incremental backups that should have been deleted from there will not be put in the backup set deletion queue until the next full is run. Make sense?
What you need to do in this case is simply reschedule a full backup to take place. This will mark the new set. From there, I recommend you delete/copy off the incremental backups previous to this tentative to be scheduled full backup.
Hope this helps you.