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