Why does some album artwork show on Entune but not others?

I've got a large music collection in iTunes on a PC. I've put this together over a lot of years. 95% is music that I either ripped from my own CD collection or stuff I downloaded in the bad ol days of Napster and the like. I do have some purchased digital files but not much.

Over the years I've put album art on to the music by grabbing jpgs and attaching them via itunes.

I use a Note4 and Rocket Player. Rocket Player uses the itunes library and makes playlists that I sync to the Note4. When I am playing a tune with Rocket Player on the Note4 I see the artwork on the phone and sometimes I see the artwork on the Entune screen. But other times even though the Note4 shows artwork I see Entune's default generic artwork. And sometimes I see artwork from the previous tune rather than from the current tune.

I think there is a file structure for these mp3's that I don't get.