Album Art Work Missing For Some mp3 & wav

I add album artwork to all of my mp3 in MP3TAG but when I play the mp3 on my Pioneer car stereo not all of the images show up. Out of 500 mp3's about 400 show the album artwork that I applied. Is there a fix for this? Thanks.

Do all the files show the correct data in MP3tag?
If so: what is the difference between those files that display correctly on the Pioneer and those that do not?
Also, you could use the search words "car stereo" in the forum search to find a number of threads dealing with car stereo problems.