I stumbled into this problem where i forgot to close "Software Management" and
left it open running in KDE. I then ran the applet from the system tray to
check for software updates it reported immediately that "your system is up to
date" even when it was not.
Two distinct problems:
   1 it does not go out on the web and check for updates
   2 it does not report the error, instead it says your system is up to date
I then closed the open Software Management gui and ran the Tray applet again
which ran normally now and it found 2 updates.
When i saw this happen i tried it on a second machine and got the same results