Please can you post the contents of /etc/update-manager/release-upgrades, it should look something like this unless it has been changed:
# default behavior for the release upgrader
#
[DEFAULT]
# default prompting behavior, valid options:
# never - never prompt for a new distribution version
# normal - prompt if a new version of the distribution is available
# lts - prompt only if a LTS version of the distribution is available
Prompt=lts
Please can you post the contents of /etc/update- manager/ release- upgrades, it should look something like this unless it has been changed:
# default behavior for the release upgrader
#
[DEFAULT]
# default prompting behavior, valid options:
# never - never prompt for a new distribution version
# normal - prompt if a new version of the distribution is available
# lts - prompt only if a LTS version of the distribution is available
Prompt=lts