Почему я не получаю & ldquo; main & rdquo; Обновления?

Недавно я обновился с Ubuntu с 12.10 по 13.04, и с тех пор у меня сильное впечатление, что я не получаю обновления для основных пакетов.

На самом деле, с момента обновления до Ubuntu 13.04 около 15 дней назад, я регулярно запускаю sudo apt-get update и смотрю, какие пакеты обновлены. Основываясь на моем опыте работы с Ubuntu, я могу сказать, что получаю обновления только для PPA (например, Novacut) и репозиториев партнеров (например, Skype). Я не получаю обновления для других пакетов.

При запуске sudo apt-get update я не получаю сообщение об ошибке. Я все еще могу установить все, что захочу, с помощью Software Center или командной строки. Вот содержание /etc/apt/sources.list:

# deb cdrom:[Ubuntu 13.04 _Raring Ringtail_ - Alpha amd64 (20130402.1)]/ raring main restricted
# deb cdrom:[Ubuntu 12.10 _Quantal Quetzal_ - Release amd64 (20121017.5)]/ quantal main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://ftp.uni-erlangen.de/mirrors/ubuntu/ raring main restricted universe

## Major bug fix updates produced after the final release of the
## distribution.
deb http://ftp.uni-erlangen.de/mirrors/ubuntu/ raring-updates main restricted universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://ftp.uni-erlangen.de/mirrors/ubuntu/ raring multiverse
deb http://ftp.uni-erlangen.de/mirrors/ubuntu/ raring-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.

deb http://ftp.uni-erlangen.de/mirrors/ubuntu/ raring-security main restricted universe
deb http://ftp.uni-erlangen.de/mirrors/ubuntu/ raring-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb-src http://archive.canonical.com/ubuntu quantal partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu raring main
# deb-src http://extras.ubuntu.com/ubuntu quantal main
deb http://repository.spotify.com stable non-free #Spotify
# deb http://apt.last.fm/debian raring main #Last.fm disabled on upgrade to raring

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.

## Major bug fix updates produced after the final release of the
## distribution.

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.


## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu raring partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb-src http://extras.ubuntu.com/ubuntu raring main

Как проверить правильность моего впечатления? Как устранить проблему?

Я попытался удалить файл /etc/apt/sources.list, а затем автоматически перезаписал его, выполнив обновление, но ничего не изменилось.