I have all this music in my itunes library, but it won't let me sync it to my iphone

I had an ipad with a bunch of music on it (That i bought off the itunes app on the ipad) so i sync it to the computer and its all on there. Then i plugged in my iphone (that also had some music on it that i bought off the itunes app on my iphone) and tryed to sync that music to my iphone. And for the life of me it won't let me do it. Any suggustions?

I don't think you can sync songs to an IPhone like you do to a IPod.