notify-send does nothing, yet, libnotify is installed

There are no special configuration files as far as I know so I would try and reinstall the package

sudo apt-get --reinstall install libnotify-bin notify-osd

In my case, the notification was not displayed because I had netbeans in full screen mode in the background.

For some weird reason, the message was getting hidden.

In my case, it only works with a zero time limit (-t 0) or with critical urgency (-u critical).

notify-send -t 0 "Nagging you."
notify-send -u critical "It's critical"