FTP Scrubbing
The FTP server has limited disk space, so once a month it is scrubbed clean.
The automated scrubber traverses each directory under /ftp/pub and /ftp/incoming looking for files that are over 30 days old. Files that are older than the 30 day period are removed.
If you have files that need to remain accessible for more than 30 days, you might qualify for an archive directory.
Read more information for the FTP Archive