I find it helpful to have the music sorted as follows:
Then make sure the files in each of the Albumname folders has a matching (including case sensitivity and & vs and type matching) Artist and Album tags.
Check for files with Album Artist instead of Artist.
I prefer to use only Artist and leave Album Artist blank except for compilations by various artists, then I use Various Artists as the Album Artist, and the actual song artist for the individual track Artist tags.
Look for wav files, not all tags seem to work properly in wav files. I found it best to stick with only flac and dsf files on Volumio. They accept all the tags I need and work properly with Volumio.
Check each album for each artist, to make sure the Artist and Album tags are consistent.
Look at the files/tracks in those mystery * album, try and see what is missing or incorrect in the tags of those files/tracks.
I had similar issues with my USB music store until I went through and carefully cleaned up the tags and file types.