Cover Art displays on old gear but not on iOS devices

The cover art for particular albums in my library does not display on new iOS devices although it shows up fine on the iMac, iPod Classic and even an ancient iPod nano. The tracks in question are in Apple Lossless format and have the cover art embedded. In the absence of available downloads for these albums (Bach Cantata Pilgrimage, John Eliot Gardiner) I scanned the artwork myself at a resolution of 600x600. No other albums in my library have this problem, including many other classical albums with self-scanned artwork. I wonder if Kirk McElhearn would have the answer?