This applies to accounts that have added additional FTP users, it doesn't apply to your main cpanel FTP log in username/password.
If additional users are not able to log in, check the following:
1. they are using the correct username format
ie email@example.com rather then just username
2. the password is correct - if you are unsure you can reset it via your cpanel login.
3. their hostname is correct - this should be the accounts domain name.