What can cause cover not to show?

I paid and downloaded (from a legitimate website) Flac tracks from a artist, certain tracks from a few of his 12" releases.
They came in zip files.
I unzipped the files using 7-zip. I then used mp3tag to embed the cover to each track and to add the cover as folder.jpg to each folder.
All of them worked fine when I added the tracks to an external hard drive (covers for each track and cover in folder showing) and connected the hard drive to my Lumin T2 streamer, except one.
The embedded cover does not show and nor does the cover which I added as folder. jpg to the folder show either.
Does anyone know why all the others show the cover but not this one track?
If there is something within this one track that stops the embedded cover from not showing, what I can't work out, is why does it affect the folder.jpg inside the folder.
How come that is not appearing aswell?
If there is a problem with this track, is there a program that can correct it?

Are these different covers?
There have been reports about progressive jpegs which some players refuse to show.
A conversion to png and back to jpg sometimes does the trick.
Then there are reports that windows does not display flac file tag data properly if one field exceeds (I think) 20 characters.
So you would have to investigate what the difference is between the files that work and those that don't.

No, I'm using the same cover to embed each track for each 12" EP release which I have in individual folders and using the cover as folder.jpg within that same folder.
I'll try the conversion you mention and see if that works.
Thanks

Also check the lengths of fields ...

I changed the cover from .jpg to .png and it worked.
Thanks again.