Assuming you are trying to move media files this was done on purpose to prevent piracy and please the content providers. Purchases can be re-downloaded from the Store and copied back to a computer. You can also right click on the iPhone and select "Transfer Purchases" if you enable the side bar in the View menu of iTunes 11.
Recovering media files from other sources can not be discussed here. The steps can be easily found by searching the net for a third party program for help.