Why will some .m4a not play on Clip Zip? How to troubleshoot?

I have a mac and rip CDs using iTunes to m4a / AAC format (standard iTunes settings). I had most of these in iTunes a long time before I got the clip zip. Now that I am adding them to my clip zip, some will play on the clip zip and some will not. They all play fine in iTunes.

Any ideas how to troubleshoot why some m4a are not working in clip zip? 
Could there be a settings difference in the files that I’m not aware of?

I think the ZIp has trouble with relatively longer AAC files, so I’d check if the files that don’t work are all longer tracks.