I was seeing the same behavior, and just figured out the problem: the current user needs to be in the "cdrom" group in order to operate the burner. This is probably the default if you're logged in as the original user you created when you installed, but may not be if you're logged in as a user you created later.
So to fix it, either:
- Log in as the original user you created when you installed Ubuntu, or
- Add your current user to the "cdrom" group.
To add a user to the "cdrom" group, log in as a user with sudo permission (such as the original user you created when you installed), open up a terminal, and run the command:
sudo usermod -a -G cdrom <username>
<username> is the user you want to add to the "cdrom" group. Then verify that the user in in the "cdrom" group:
It should show "cdrom" in the list of groups.
Log out and back in as
<username> (this is important, or the desktop environment won't pick up the new group membership), insert a blank CD, and you should be set.