Lately FTP has been a problem for a lot of hosting providers. Especially since the recent influx of Gumblar-related attacks and FTP exploits, some hosting providers are now considering changing their FTP ports as an added measure of security. If you're on a cPanel server you have a couple extra steps to go through, but it's a rather easy change.
1) Edit /etc/pure-ftpd.conf and look for the following line:
2) If necessary, add the new FTP port to your server's firewall
3) Edit /etc/chkserv.d/ftpd and change the port, which is the first comma-separated entry to the right of '=' on the line in the file, to match the port you put the service on.
/etc/init.d/cpanel restart (restarts tailwatchd/chkservd)
Edit /etc/proftpd.conf and change:
Then make sure to test to make sure FTP is working before notifying your users!