i have changed the "server" from the Main Server to a server in Japan via "Select Best Server"
then after that, I could not anymore check the empty boxes that said Main, contrib and non-free.
furthermore, on the "Updates" tab, the boxes "proposed updates" and "security updates" are un-checkable too
i am wondering what happened. I do not want to have a broken synaptic package manager running on Debian 6.03