Time Machine will delete snapshots over time. It will retain hourly backups for the past twenty four hours, daily backups for the past month (~30 days) and weekly backups for as far back as it can until the backup disk is full.
Presumably the reason that your snapshots are disappearing is that the August backups have begun to reach the one-month threshold and are now being thinned from daily to weekly.
This is normal, and, to the best of my knowledge, unchangeable behaviour for Time Machine. Time Machine is trying to be an approximation of a continuous backup and thus thins out older backups in order to give more room to allow more recent backups to be more frequent, which helps to sustain the illusion.
If, however, you'd rather have unimpeachable snapshots of your computer, use a different program, such as SuperDuper, rsync, or Disk Utility to make a copy of your hard drive from time to time. That way, it won't be thinned, and you can also do other stuff, like making it bootable.