New dependency normally is automatically installed, kept back means dependency is unresolved, which could be due to
updates instead of main)Does it work eventually if you keep pressing the button several times?
I had that problem but in my case it seemed to work if pressing the button a second time. See:
But would any of the reasons you listed explain why the “kept back” issue could be fixed by simply doing “apt install” for the package? I know that has worked for me several times.
Oh, I hadn’t seen that bug report. I’ll check again, but as I recall it sometimes failed on more than one press.
I think @ruff is closer to the mark. I did reboot and did update and upgrade a few times and eventually the problem went away. I wasn’t systematic enough to know what did the trick. That error (on any distro) for me only occurs rarely. So it’s one of those mystery annoyances that I never looked into in more detail.
So now I’m waiting for the next update in order to try to do the update via the GUI …
How embarrassing. 
You are right.
So approx. 90 days after getting my phone, I have for the first time applied an update via the GUI. LOL. Maybe this is a deliberate feature so that you don’t accidentally restart the phone i.e. “bum restart”.
I won’t mark that as “Solution” since my question was a hijack.
When in doubt, keep hitting the button (any button) until it works. 
I doubt the button on my oven that says “Stop Time.” But I will not touch it.
in PureOS on LMini you can set the internet connection to be ‘metered’. if you do so you will notice that applying updates from the Software Store side is restricted for data reasons … you can however force it to update and upgrade each time with :
sudo apt update
sudo apt upgrade
sudo apt full-upgrade