Ubuntu's Notify OSD was great idea, most of functionality unfortunately wasn't implemented. It seems will never be completed.

If you are still using Notify OSD it that and replace with with Dunst .

Dunst advantages over Notify OSD:

• Can show more than one message ⚠
• Minimalistic
• Highly configurable
• Message stacking
• Group duplicated messages
• Close messages on click
• Notifications with critical urgency ( notify-send -u critical test ), don't dissapear automaticaly and will be only closed on click
• idle_timeout so you don't miss notification if you are not near computer
• Custom rules for certain applications
• History
• Supports urls
• Works fine in multiple monitor configuration

See sample config .

sudo apt-get remove notification-daemon notify-osd
killall notify-osd
sudo apt-get install dunst
# try it:
notify-send -u critical test