I commented in this thread, but decided to open a new thread also.
For some reason, software updater hasn't showed up after I upgraded my Lubuntu from 13.04 to 13.10. I think it is running based on the hard drive activity every day, but the dialog (update notifier) just doesn't appear. If I start it manually from System Tools -> Software Updater, it opens normally, displays available updates (both security and other) and installs them correctly. Now it says that my system is up to date, but I cannot trust it to appear when new updates are available. What might cause this?