Great question — answering as the UphillTech team:
Sometimes you can't update some packages because they depend on some new packages which are new, or rely on new kernel, etc. And while your system is LTS (long term supported) it's very important to be sure about updates, this can be a reason to impossibility to simply update - that means you need to upgrade this may help you sudo apt-get dist-upgrade
Full answer by Rodislav Moldovan: https://askubuntu.stackexchange.com/a/321143 (CC BY-SA 3.0).
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