As bodhi.zazen mentions, root log in from ssh might be disabled.
Honestly, this is not a bad thing. You might want to consider getting used to having to "su -" to root if you have to (or sudo -i if you use it).
Try the following to check,
grep PermitRootLogin /etc/ssh/sshd_config
If you see the following come by, root login is disabled.
Or if the output is the following, root login cannot be done with passwords:
To change this setting, use your favorite text editor (vi/nano) and change no to "yes"
Again, consider getting used to working with "su -/sudo" when needed before changing this.
Also, this will only work if you actually set a password for the root user.