By default SSH doesn't allow root logins for Ubuntu. To enable root login run the following command:
sudo nano /etc/ssh/sshd_config
Under the Authentication section find the following:
PermitRootLogin prohibit-password
And comment it out as follows:
#PermitRootLogin prohibit-password
Next add the following directly below the commented out line from above:
PermitRootLogin yes
Finally, restart SSH with the following:
sudo service ssh restart