I'm using Ubuntu 11.04. The update manager once found the new release 'oneiric', and still shows up this screen when I log on use ssh:

Welcome to Ubuntu 11.04 (GNU/Linux 2.6.38-14-generic x86_64)



 * Documentation:  https://help.ubuntu.com/



0 packages can be updated.

0 updates are security updates.



New release 'oneiric' available.

Run 'do-release-upgrade' to upgrade to it.



Last login: Wed Apr 25 16:22:48 2012 from ***

But I didn't upgrade then, and change my apt sources. And now I cannot upgrade to 'oneiric'. do-relase-upgrade shows:

$ sudo do-release-upgrade

Checking for a new ubuntu release

No new release found

$

And apt-get dist-upgrade shows:

$ sudo apt-get dist-upgrade

Reading package lists... Done

Building dependency tree

Reading state information... Done

Calculating upgrade... Done

0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

$

I can successfully update all my packages.

File contents of source.list:

$ cat /etc/apt/sources.list

## See sources.list(5) for more information, especialy

# Remember that you can only use http, ftp or file URIs



deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty main universe restricted multiverse

deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty main universe restricted multiverse

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty-security universe main multiverse restricted

deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty-security universe main multiverse restricted

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty-updates universe main multiverse restricted

deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty-updates universe main multiverse restricted

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty-backports universe main multiverse restricted

deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ natty-backports universe main multiverse restricted





# deb http://ubuntu.dormforce.net/ubuntu/ lucid main universe restricted multiverse

# deb-src http://ubuntu.dormforce.net/ubuntu/ lucid main universe restricted multiverse

# deb http://ubuntu.dormforce.net/ubuntu/ lucid-security universe main multiverse restricted

# deb-src http://ubuntu.dormforce.net/ubuntu/ lucid-security universe main multiverse restricted

# deb http://ubuntu.dormforce.net/ubuntu/ lucid-updates universe main multiverse restricted

# deb-src http://ubuntu.dormforce.net/ubuntu/ lucid-updates universe main multiverse restricted



# CDROMs are managed through the apt-cdrom tool.

# deb http://archive.canonical.com lucid partner

# deb http://archive.canonical.com lucid-security partner

# deb http://archive.canonical.com lucid-updates partner

# deb-src http://archive.canonical.com lucid partner

# deb-src http://archive.canonical.com lucid-security partner

# deb-src http://archive.canonical.com lucid-updates partner





#medibuntu repo

# deb http://packages.medibuntu.org/ lucid free non-free

# deb-src http://packages.medibuntu.org/ lucid free non-free



# deb http://extras.ubuntu.com/ubuntu maverick main #Third party developers repository

deb http://mirrors.sohu.com/ubuntu/ natty main restricted multiverse universe

deb-src http://mirrors.sohu.com/ubuntu/ natty main universe restricted multiverse #Added by software-properties

deb http://security.ubuntu.com/ubuntu/ natty-security universe main multiverse restricted

deb-src http://mirrors.sohu.com/ubuntu/ natty-security universe main multiverse restricted

deb http://mirrors.sohu.com/ubuntu/ natty-updates universe main multiverse restricted

deb-src http://mirrors.sohu.com/ubuntu/ natty-updates universe main multiverse restricted

File contents of /etc/update-manager/meta-release:

$ cat /etc/update-manager/meta-release

# default location for the meta-release file



[METARELEASE]

URI = http://changelogs.ubuntu.com/meta-release

URI_LTS = http://changelogs.ubuntu.com/meta-release-lts

URI_UNSTABLE_POSTFIX = -development

URI_PROPOSED_POSTFIX = -proposed

What may be the problem of this?