What is making one album not show artwork when using a streamer?

I have a collection of albums in Flac, which I am going through to make sure the metadata is correct.

I've come across one album which I have done exactly the same as the rest of the albums (using mp3tag) but it won't show the albumart on my streamer even though I can see it when using mp3tag on my Windows laptop.

Any idea what is causing the albumart not to show when all other albums are fine?

No-one can look over your shoulder and see what could be the cause if everything is fine with all the files.
So you have to look for differences: a check of the integrity of the files is always a good start, compare the length of data in the fields of files that work and the ones that don't. And talking of sizes: the size of the embedded picture may upset your streamer.

I'm sure I've read about someone else having the same problem months ago.
I've searched but can't find it.
If I remember correctly 'Convert' was used but I'm not sure which one of the five was chosen?
Any idea which one could be tried that would change the tag information that could help?

For which purpose?
Convert>Filename-Tag imports Text from the filename to tag fields
Convert>Tag-Filename renames the file with tag data according to what the user sets.
Convert>Tag-Tag sets data to a tag field.
Convert>Text file-Tag import data into (a) tag field(s).
Convert>Filename-Filename rearranges the filename.

So: could you show a screendump of the extended tags dialogue of a file that works and one of a file that does not?

Perhaps it was for another problem? I can't remember.