1. Make sure you have "Authorised the Computer" logged in with the same username and password used for purchases on your iDevice.
2. Delete the iTunes data files and start again (located under documents -> music -> iTunes)
3. Since Apple fail to allow customers the right to keep there data and wish to loose the market over time from people who are sick of having so much control and not enough support and care, I suggest you use Wondershare, wonderful software however it comes at a cost. Wondershare iMate and Wondershare Transpod do a wonderful job. Hopefully apple will make a change and allow iDevice to iTunes transfer. Help me provide feedback to apple: www.apple.com/feedback and be gone of this horrible control. For now, thank you Wondershare doing what Apple failed to do. (as of March 2012)
Let me know if you have found another method to this madness,