Comment 14 for bug 1224098

@Remten: unfortunately, we only modify the main lightdm configuration file. If you install other Ubuntu flavours after installing nvidia-prime, it might not work.

You can try the following steps:

1) Remove the xubuntu settings package (also pay attention to the other packages that this removes):

sudo apt-get --purge remove xubuntu-default-settings

2) Reinstall xubuntu-default-settings:

sudo apt-get install xubuntu-default-settings

3) Reinstall any packages that were removed when you uninstalled xubuntu-default-settings

4) Make sure that the alternative is set to Xubuntu:

sudo update-alternatives --config lightdm-gtk-greeter-config-derivative

(and select the alternative for Xubuntu)

5) Finally reinstall nvidia-prime:

sudo apt-get install --reinstall nvidia-prime