Album art work is a constant question on people's computers and without getting technical, its all about time .
new CD's have all the metadata on the cd directly hence the artwork is instantly there. older tracks and CD's the software takes time to find and it depends when it trauled for metadata.
example when you create a usb by copying old files (music) inside that file will be a system information file, sometimes hidden this is the metadata for artwork etc.
iTunes metadata can only be read by Apple devices so useless.
if you can, save all your music in your Music file and leave it there if there is artwork out there it should find it, dependant on correct spelling and album title. when you then load your USB for the car, all the system info files come with it.
of course this depends on the computer gods having a good day