upgrade from 14.04 to 15.04?

[Luca Ferrari]

Hi all,
I'm trying to do the upgrade but cannot succeed, and cannot even
upgrade to 15.10.
Here it is my version after a full dist-upgrade:

% lsb_release -a
                                       ~
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 14.04.2 LTS
Release:        14.04
Codename:       trusty


and here is the effort to upgrade:

% sudo do-release-upgrade -d
                                       ~
Checking for a new Ubuntu release
No new release found
% sudo do-release-upgrade -c
                                       ~
Checking for a new Ubuntu release
No new release found


What am I missing?

Thanks,
Luca