Debian buster on Lenovo G550, no longer updates or upgrades

I installed Debian buster 10.1 on the laptop (and also on a desktop) in March 2020. All went fine as time went on -- each time a new revision of buster came out, I ran "apt update" and then "apt upgrade", and the desktop and the laptop both updated and then upgraded buster. Until Debian 10.9. Since then, when I run "apt update" and then "apt upgrade" on the laptop, I may get an occasional upgrade to e.g. Firefox, but Debian itself does not update (or upgrade). The other Debian buster system -- the desktop -- updates and upgrades fine, and is as of this morning on 10.11.

For a while I thought that maybe the reason for the difference in behavior was that the 10.10 and 10.11 upgrades perhaps were "not for laptops" ... but now I am thinking that something else is wrong ...