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?