What are the possible reasons a kernel security update would fail to occur during unattended-upgrades? - Ask Ubuntu

i have setup unattended-upgrades along lines of ubuntu automaticsecurityupdates doc. however, had not updated latest linux kernel, "security" update. possible reasons?

i ran apt-get update && apt-get upgrade && apt-get dist-upgrade manually today, , upgraded kernel successfully.

/etc/apt/apt.conf.d/20auto-upgrades

apt::periodic::update-package-lists "1"; apt::periodic::unattended-upgrade "1"; 

/etc/apt/apt.conf.d/50unattended-upgrades

unattended-upgrade::allowed-origins {         "${distro_id}:${distro_codename}-security"; //      "${distro_id}:${distro_codename}-updates"; //      "${distro_id}:${distro_codename}-proposed"; //      "${distro_id}:${distro_codename}-backports"; };  // list of packages not update (regexp supported) unattended-upgrade::package-blacklist { };  unattended-upgrade::automatic-reboot-time "05:00"; unattended-upgrade::automatic-reboot "true"; 

if /boot partition out of space new kernel, fail. apt-get autoremove'usually clears unless have kernel stuck 1/2 way through install. in case need manually make room , run apt-get -f install complete install.

i wonder if in situation didn't have kernel stuck installing , perhaps cleaned space in process. think i've seen once, isn't common. breaks congrats!