Problem when syncing iPhone 5s

[abbasi]

[FONT=arial]Hi,[/FONT]
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[FONT=arial]I downloaded a free app from appstore by iTunes. Just like before, I went to app part and clicked the install and then apply. iTunes was syncing, the steps were 5 and iTunes went until 3 and this message appeared. [/FONT]
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[FONT=arial]" The iPhone .......could not be synced, because this computer is no longer authorized for purchased items that are on this iPhone.[/FONT]
[FONT=arial]To authorize this computer for items purchased from iTunes store, choose store > Authorize this computer."[/FONT]
[FONT=arial] [/FONT]
[FONT=arial]So I try to authorize the computer like that way, I enter my ID and password and then click Authorize. Now my compter is authorized.[/FONT]
[FONT=arial]But when I reclick on apply to sync, those messages will appear again and again. What is the problem and can I solve it please?[/FONT]
[FONT=arial] [/FONT]
[FONT=arial]Thanks[/FONT]