notify-send not working under ssh
I think you're confusing the various technologies and how they work. I wouldn't expect that the notification daemon from one system could send messages via SSH. Setting the $DISPLAY is how X11 sends the output from an application to another for displaying purposes, but the notify-send is sending an actual message to the notification daemon. This message is send using the libnotify library.
libnotify is a library that sends desktop notifications to a notification daemon, as defined in the Desktop Notifications spec. These notifications can be used to inform the user about an event or display some form of information without getting in the user's way.
Source: https://developer.gnome.org/libnotify/
Per app approach
One method for joining the notify-send messages to your local system's notifier is to use a approach as outlined by this blog post titled: IRC notifications via SSH and libnotify. This approach would need to be customized per each type of notification that you'd want to tunnel back to your local notifier.
Tunneling libnotify over SSH
For a more general solution libnotify-over-ssh may be more what you're looking for.
This is a client server perl script I wrote so that my server could essentially send libnotify messages to my local machine. I use this mainly with weechat but has a feature to make it more general. When calling the client with the weechat tag the server checks the name of the current focused window. If it starts with weechat, notifications are suppressed if not notify-send is called.
Displaying on the remote server
If on the otherhand you're simply trying to use notify-send to display messages on a remote server that you've used ssh to connect to, you'll likely need to follow one of the suggestions that was made in this Q&A titled: Using notify-send with cron. Even though several of the answers suggested that this was unnecessary, I had to do the following as others mentioned in comments on my Fedora 20 system using Cinnamon as my desktop to get things working.
To get notify-send working I had to set this variable with the appropriate value from the remote system's desktop environment.
$ export DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-wzrbFpDUZQ,guid=82e5bffe1f819506faecc77a53d3ba73
On my system I was able to make use of a file that's maintained for this exact purpose.
$ ssh me@remote
$ source ~/.dbus/session-bus/6a34f24f9c504e3f813bc094ed0b67af-0
$ notify-send "hi"
NOTE: The name of the DBUS file will change from session to session.