Same problem with a Class 4. But I figured it out.
It’s because iTunes writes to ID3 2.2 instead of ID3 2.3. I managed the problem tracks right inside iTunes by right clicking the track or tracks, choosing CONVERT ID3 TAGS, selecting 2.3 from the drop down menu, and clicking OK to save. Presto. The art displays correctly. There’s no need for a separate tag editor application.
I was baffled because most art displays fine and they are all written by default with ID3 2.2 tags.
