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Hi, I now have this problem too, but you do not need to delete all info from your iPad/iphone, when your iPad/iphone is plugged in and connected to iTunes, click on iPad/iphone as if you were going to sync then click on 'on this ipad/iphone' part on the right hand side tab, scroll down all your music and see which ones are in great with a circle next to it on the right left hand side.
If you have made movies, music videos etc and brought them in, try changing it to movies or home movie under get info under that track and sync in movie section and they should be ok, however, I'm still trying to suss out why the normal track will not play. 🙁