Have you looked to see if there is a tick in the check box next to the songs in itunes? If not it wont play.
In the case of older formats like some MP3's some might sync okay but some might not even though they could be all the same file format.
If there is a "Syncing symbol next to the ones that did not copy over this probably will be the case" You can then fix this problem by converting the music files that are in the playlist. If you select them in iTunes and right click then select "Create AAC version" they will make a .m4a file which will sync across fine.
I have since found that changing the file format (although works) is not necessary. All you need to do is change any info of the file so that the song ID in Itunes gets recreated and it recognises it to sync it.
Change the artist name, or albulm name or anything on the info tab on the "Get info" option for each song.
Select all the songs you need to update and change a field, even if it is to put in a comment called "updated"
Much simpler than my first suggestion.