I got an email that Ubuntu Mate 21.10 was available, so I started up a Mate VM. As soon as the system booted, a prompt appeared on my desktop:
"A new version is available would you like to upgrade?" I clicked the OK/Yes button. The prompt disappeared, but nothing happened.
So, then I started Software Updater. It showed me a small number of available updates were available, so I thought perhaps that was what caused the original upgrade not to work. I applied the available updates. When finished, without further action on my part, Software Updater then showed me a message that said:
"The software on this computer is up to date. However Ubuntu 21.10 is
now available (you have 21.04)."
Again, I click the Upgrade button, and again, nothing happens. I can now repeat this Software Updater process repeatedly, with the same results. I looked at many different system log files: syslog, kern.log, dmesg, dpkg.log, Xorg.0.log, apt/history.log. I can't see anything obvious that explains why these upgrades just disappear without doing anything.
I tried rebooting to see if that might change the outcome. Upon reboot, I no longer see the prompt on my desktop on system startup. However, using Software Updater produces the same result described above.
Before I drop to the command line and start with do-release-upgrade, since I'm in a repeatable failure mode, I wanted to offer up the opportunity to debug why this is happening. Since I can't find any applicable messages, I'll need some guidance on what to do next.