Try running '/usr/lib/ubuntu-release-upgrader/check-new-release-gtk' in a terminal and see if any messages are displayed. For future reference release upgrade log files are stored in /var/log/dist-upgrade.
Try running '/usr/lib/ ubuntu- release- upgrader/ check-new- release- gtk' in a terminal and see if any messages are displayed. For future reference release upgrade log files are stored in /var/log/ dist-upgrade.