Darren has a brand new iPhone 6 and he's having trouble syncing his music to it from his MacBook Pro. He's missing several hundred songs and it keeps looking to sync, wearing down the battery. Doctor Mom says that if he has a music file that he's made himself, it could be blocking the sync. There's also two different bugs that can delay the sync. Here's a MacRumors link on them - http://forums.macrumors.com/showthread.php?t=1786762. If there's a bad track, it will cause it.
Leo recommends paying the $25 a year fee for iTunes Match, which will put all of his songs up in the cloud. It will match everything, even music that he's ripped from CDs, and it will replace them with high res copies in the cloud. Then he can delete them from his Mac and restore from iTunes Match. He'll then be able to re-download them, and have fresh versions that are better.