If you are having trouble with your FTP login even though you entered the correct username and password, there is a high possibility that this problem was due to SELinux.

I've encountered this problem on my CentOS 6 box and after disabling SELinux, the issue is resolved! To disable SELinux (for CentOS 6):

1. Type nano /etc/selinux/config
2. Change value SELINUX=disabled
3. Reboot your server

Hope it helps! It took me hours to get this solved.