Using Ubuntu 12.04, I cannot see any update from my update manager. It gives me the message "check your internet connection"
If you are able to use the internet, and your network is working, then try this: Open Update Manager, press Settings button and on te first tab, change the server where it says "Download from: ". I had the same issue, and for me it was automatically set to Germany. I selected Main server, and it's fine now: