I have tried all the recommendations about art file size, DPI, baseline, and placing art renamed to folder.jpg into the album folder. However, some album art stubbornly refuses to display if the album is on an external chip, but will display fine if the album is on the internal memory. I have copied and deleted the affected files back and forth from internal to external several times and the result is always the same.
All of the music was ripped to 320k MP3s from CDs with the latest version of iTunes. I downloaded separate art, edited it to an acceptable copy in PhotoShop, and added it to the files. None of the music uses art downloaded from iTunes. All the art displays in Windows Media Player, my home music server software, and iTunes. The external chip is a SanDisk Ultra 32 GB class 10.