In case you're wondering, I've already been through a few questions on the same topic here, and a bunch of threads on forums here and there. No solution seems to apply to my case. I have 11.10 currently installed on my machine. Some time ago I received regularly notifications of upgrade to 12.04. The release upgrade button showed up regularly in the Update Manager. But so far I have ignored all of this because I wanted to get back home before updating the system (just in case I needed a second machine to repair an unsuccessful upgrade). So every time the notification window showed up I've clicked on "Ask me Later". Now that I want to upgrade: 1. The button is no longer in the Update Manager 2. sudo do-release-upgrade -d returns "No new release found" 3. sudo update-manager --dist-upgrade returns "Your system is up-to-date"
Apparently I can install normal updates, so I'd rule connectivity issues out.