When syncing my iphone it doesn't put any music purchased on my phone onto my computer, says its not aurhorized computer. When I go to store and try to aurhtoized says is already an authorized computer. What do I do to correct

I have purchased music on my iphone but when I sync it won't put this into my itunes account or onto my computer, it says my computer is not authorized and to go to the store and authorize it. When I go the store and click on authorize computer it says it is already authorized. I have unauthorized and re authorized my computer but still not recognized, what do I do. I want my new music on my computer so I can load it on my ipod.