How are you doing the upgrade?
This is suggested on their site
To upgrade from Ubuntu 11.10 or Ubuntu 10.04 LTS on a desktop system, press Alt+F2 and type in "update-manager -d" (without the quotes) into the command box. Update Manager should open up and tell you: New distribution release '12.04' is available. Click Upgrade and follow the on-screen instructions.
Perhaps you need to be hardwired to a connection instead of doing it over wireless
Good luck to you!