why are some songs from albums unavailable

like for instance im trying to sync the album almost here by the academy is... i had the album on my laptop and synced it to xbox music. it synced most of the album but not slow down. i deleted the album and put it back straight from xbox but it still will only let me download the songs except slow down. ive noticed this happen on many albums in my library and its the only thing keeping me from using my windows phone and laptop perfectly. honestly it worked so much better to use zune and sync local playlists.