Why am I being prompted for a password when using an SSH key?
The most common cause for this is incorrect permissions on the 'authorized_keys' file which can be found in the user's home directory where '$user' represents the user you are attempting to connect to the SSH service with:
The permissions on this file should be '600', the permissions can be updated using the following command:
chmod -v 600 /home/$user/.ssh/authorized_keys
We have further information on updating file permissions here.