Testing for available updates does not take place

update-notifier is configured to check for available updates once a day. It never reports any. Whenever I manually check for available updates every couple of days by launching update-manager and updating package lists, pending updates are found. Apparently this system fails to update package lists automatically in the background and thus fails to detect outstanding security patches and other updates.

This situation has been unchanged since it first occurred on Natty, whether I was using the gnome2 shell, unity3d or unity2d doesn't seem to make a difference. In case I am not the only one whom this happens to then this should be handled with high priority since an operating system which is unable to make available security updates to its users (many of which will not realize this is the case) is not overly convincing.

I'll happily to provide any further information which may be needed and happy to read up on documentation when pointed to any.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: update-notifier 0.117ubuntu3.1
ProcVersionSignature: Ubuntu 3.0.0-13.22-generic 3.0.6
Uname: Linux 3.0.0-13-generic x86_64
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
Date: Sun Nov 20 11:58:11 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
ProcEnviron:
 PATH=(custom, user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: update-notifier
UpgradeStatus: Upgraded to oneiric on 2011-10-21 (30 days ago)