SetAuthCookie problem

[Cheung Wang Tin]

I have a login page which with the following code:
FormsAuthentication.SetAuthCookie(UserID, False)
'''''For testing''' lblTest.text=context.User.Identity.IsAuthenticated
Response.Redirect("Default.aspx", False)

When loading the login page alone, the login function work fine.
But when the login page was included in other web site's frame, it can not login. The value of context.User.Identity.IsAuthenticated equal to "False" even I provide the correct username and password several times.

Also, when the web site's IP is the same as the login page's IP, it can login.

Does anyone have idea about it? Thank you.