Try looking at the ones that actually work first.
Write them down and take them into mp3tag. See what size of cover you use in relation to pixels/kb of the artwork and if there are any differences in the files that don't work. Sounds like a memory issues with the player so pixel size/file size may be the problem.
See this post on how to set up a column and how to use filters [F3] to narrow down the files with large file size of cover art. Unfortunately there is no way to filter the pixel size in mp3tag as yet.