Itunes won't copy music to my iPhone?

I have itunes 11 and my iphone is iOS 6.
I always manually copy (click and drag) my music to my iphone, and it works with some songs and others it doesn't.
I have 3 or 4 songs I really want on my phone but won't copy. And I've tried syncing but that doesn't work either.

If you are jailbroken then you can download music directly on iphone and transfer music to library using Bridge available in cydia

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