Why do my most recent iTunes music purchases not sync to my iPhone?
Solution 1:
I had this exact same problem and found the answer I was looking for within one of the Apple communities. Simply go into iTunes store and scroll right to the very bottom. There amongst the small print you'll see an option for purchased. Click on there and you will see a list of all songs/albums and you can choose to either download all or individual tracks. Once downloaded they appear in your library as normal. Don't ask me why they don't appear straight away, but this resolves the problem of those not transferring when syncing
Solution 2:
From my own experience after updating to iOS 7 I've had the same problem. I fixed this by doing a hardware reset
Hold the powerbutton and the homebutton together until the screen turns black and the Apple logo appears on the screen.
After that, I was able to sync my music correctly. Also check if you have the latest iTunes version as Apple might have put a fix for this issue in a new version.
If that does not work, log out from iCloud on your iDevice and log back in. That fixed the issue for some other people around, it might help you out too!
Please also check out This Official Apple Page, as it's filled with 54 pages of this same problem.
Solution 3:
Are there any music which you didn't purchase using your current Apple ID? Your iTunes should be authorized with the Apple ID the songs are bought. In case you're not sure, connect your iPhone to your PC/MAC, press "Back-up iPhone" (in the summary page of your iPhone) in iTunes. It will show you what is preventing iTunes from syncing your iPhone.