When I get notifications, they remain, permanently, even after the event has been resolved. They persist through ending the session and beginning again, although not through a reboot. The example I have attached is a system update available notification, but earlier, I had different notifications. Hovering over them makes them transparent but doesn't reveal a way to remove. The notification menu in the system tray says there are notifications.
dkesh@bran:~$ lsb_release -rd
Description: Ubuntu 15.04
Release: 15.04
dkesh@bran:~$ apt-cache policy kubuntu-desktop
kubuntu-desktop:
Installed: 1.323
Candidate: 1.323
Version table:
*** 1.323 0
500 http://us.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages
100 /var/lib/dpkg/status
Confirmed.
Particularly, in my case notifications about Wi-Fi don't fade away (losing connection, succesful connection, 'can't scan while already scanning')
Temporary solution - restart plasma:
Alt+F2
killall plasmashell
plasmashell
Then the notifications 'catch up', i.e. the ones that should have appeared after the one which was stuck, appear for a brief moment and the fade away normally