GFS Retention - Too much Backups are deleted

Hi,

I'm using a GFS Backup-Scheme with the following retention / keep backups rules:
- Daily: 30 days
- Weekly: 8 weeks
- Monthly: indefinitely (wich I would like to clean up manually from time to time)

I have noticed already several times, that sometimes for some reason even the monthly backups were beeing deleted. Today I have noticed that there is only one full backup from 23-07-2013 left, but this backup-task exists since at least 8 months and a few weeks ago there where many more full (and diffential) backups.

Is anyone other also facing these issues or can help me?
Thanks in advance!