Set the explorer to show hidden files and then check if there is a file called folder.jpg in the folder. If this then also shows the expected cover, then there you are: the cover can be found in that external file, it is not embedded.
You can embed the external cover art with a single action "Import cover from file".
Enter as filename: folder.jpg
No, this isn't as simple as that. Thank you for your suggestion, but I'm way ahead of you. I always have explorer set to show hidden and system files. And I know MP3tag inside and out for the mot part. I know very well how to add artwork via a jpg or png, that's pretty elementary. And again, thanks, but that is not a solution for this problem.
The artwork is embedded in the MP3 file. Really. I know what that means. There are no additional image files, jpg or otherwise, in the folder along with the MP3's. In explorer, when I select one of the MP3's, I can see the image in the details pane (because I have explorer's view set to show the detail pane, which is on the right hand side of explorer). If I set explorer's view to show medium icons, or tiles, I can see the image display on each and every individual file. However, if I set explorer to display large or extra large icons, the image displays on only SOME of the files - not all of them.
Not that I'm going to lose sleep over this but it truly is an unusual problem that defies any conventional means of fixing with MP3tag.
Is that so?
You could check what kind of tags you read. Perhaps the covers are embedded in APE tags but you don't read them?
Or: Create a column with the value %_covers% and see how many are shown.
Then: check the integrity of the files with mp3val, mp3diags or foobar2000.
And if iTunes also does not show any embedded covers ... what about a left-over thumbs.db?
Is it possible to embed another cover with MP3tag? What does MP3tag then show?
No thumbs.db file present in the folder. I'm set to read APE tags. The tag column shows ID3v2.3 (ID 3v1 ID 3v2,3). No indication the tag is bad. I use the %_covers% column regularly (as well as many other column I've added) - no indication cover is present there. Yes, embedding another cover is the easy work around. Comparing the newly embedded files with the originals, I see no differences in what MP3tag shows.
Just a mystery I guess. Thanks for your suggestions.
The mentioned utilities check (among others) the file integrity. There have been cases where tags were stacked on top of each other so that a more or less random result was displayed. You could detect such errors with the utilities but usually not with MP3tag.