Connersw, thanks for your reply.
You asked: "What other program are you using that you can see the cover art embedded in the tracks? "
In Explorer, in my Music library, I can right-click on any track and get tabs for "Audio Properties" and for "ID-Tag." (I think the dBpoweramp software gives me those tabs.) Under either of these tabs, I can see the Cover Art displayed. And, for those tracks that I ripped on my own with dBpoweramp, I also have a Folder.jpg file along with the tracks. And the Cover Art is importing into MC just fine for those tracks.
The 3rd party service that I used to rip 50 of my CDs uses GD3 software. And those are the tracks for which MC is not reliably importing the Cover Art. I can see the Cover Art by right-clicking on the track in Explorer, as described above. But, for about 1/3 of the tracks, the Cover Art is not making its way into MC.
You suggested the following settings: "In Options -> File Location -> CoverArt make sure you have Also store image in the file's tag checked." and
"You can also have MC get cover art for you on import: Options -> Library & Folders -> Configure auto-import -> Tasks -> check Get cover art."
I had, in fact, already checked both these settings.
You wrote" "For the one file where the cover art does not match, you probably have different cover art in the file vs what the folder.jpg image is. Choose which one you want and fix accordingly."
Unfortunately, for the tracks ripped by the third-party service, there is no separate Folder.jpg. The Cover Art is embedded in the tracks, only. And for at least a couple of discs, the correct Cover Art is embedded with the tracks, as I can see in Explorer, but a different Cover altogether comes up in MC. (In those instances, the Cover displayed by MC is for the same Artist but a different CD altogether.)
As you can see, I'm stumped.