Not all local files sync to my galaxy s3

[Mike_alive]

Hey folks,

I have a weird problem. Just bought an audio book, transferred it to my pc (mp3-format) via iTunes and created an offline playlist in spotify (PC). I started to sync this new playlist to my android phone to be available offline, but round about 40% of the tracks simply won't sync. 

The funny thing is that not a whole CD can't be synced, but that random tracks just appear grey and "are not available", even though I can play them on my PC.

Anyone who can help me out here?

Thanks in advance!

Mike