Fedora 17 64bit : can’t update via yum ( the result is always “no packages marked for update” )

I have one desktop machine and one laptop machine and I have installed Fedora 17 64bit on both boxes.
Whenever I try to update packages with “yum update” on the desktop machine , the result is always “No Packages marked for Update”
I , however , can update packages with “yum update” on the laptop machine.

[root@f17-64 ~]# uname -ri 3.5.5-2.fc17.x86_64 x86_64 [root@f17-64 ~]# cat /etc/fedora-release Fedora release 17 (Beefy Miracle)

[root@f17-64 ~]# yum update -y Loaded plugins: langpacks, presto, refresh-packagekit No Packages marked for Update

After issuing “yum clean all” , I can update packages.

[root@f17-64 ~]# yum clean all Loaded plugins: langpacks, presto, refresh-packagekit Cleaning repos: fedora google-chrome updates Cleaning up Everything

[root@f17-64 ~]# yum update -y <snip>  vlgothic-p-fonts.noarch 0:20120928-1.fc17                                       wodim.x86_64 0:1.1.11-14.fc17                                                   xulrunner.x86_64 0:16.0.1-1.fc17                                                ykpers.x86_64 0:1.8.0-1.fc17                                                    zeitgeist.x86_64 0:0.9.5-1.fc17                                                 zeitgeist-datahub.x86_64 0:0.9.5-1.fc17                                        Complete! [root@f17-64 ~]#

Seen from /var/log/yum.log , I can not update packages after Oct 12th.

[root@f17-64 ~]# egrep "Oct" /var/log/yum.log | awk '{print $1,$2}' | uniq | tail -2 Oct 12 Oct 18

It seems that there is any problem with yum-related data such as metadata , headers etc...