Authentication in asp.net 2.0

Hi all,
I developed a website using asp.net 2.0 with form authentication.
Everything working ok on my PC, then, i upload my website to server, and there is a problem, after logged in
HttpContext.Current.User.Identity.Name return blank
but it working ok when i run my website local on server.
Please help me!
PS: i am not sure about my english, sorry about that.
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