My ipod touch won't synch with itunes

Hello,

I have an ipod touch 3g 16 gb. Suddenly he wouldn't synch with itunes anymore. Itunes simply don't recognize my ipod touch. It was weird because my ipod classic 120 gb works just normally. It appears in itunes and it synch just fine. Well then, I bought a new ipod touch 4g 8 gb. And the new ipod touch has the same problem as the old ipod touch and the classic one still works fine. Now the problem is why don't itunes recognize and synch with my 2 ipod touch? I have tried restart the pc and restart/reset the ipods. Also I have installed the newest update and software from itunes and the newest software for the ipod. What can I do?

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