FTP is the fastest and most efficient way to move files from one computer to another. There are many possible scenarios as to why some FTP servers are not accessible. The FTP server is inaccessible by anonymous login. Entry into the site requires a user name and password. The FTP server is overloaded.
Many sites only allow access to a certain number of users at any given time.
The FTP server has crashed or has been intentionally turned off. The FTP server is behind a firewall, and your IP address ranges are being blocked. There are many possible scenarios as to why some files cannot be downloaded. The file is contained within a directory that is inaccessible by anonymous login. A ratio site is one that requires the user to upload files to the server before receiving permission to download files from that server.