Problem in authentication in live

Hi All
    I have used .net 3.5 framework and Sql Server 2005. The project is in live.I have used "Forms Authentication".Consider there are 15 users.Some users cannot view the login screen itself.when they type the URL the window authentication screen appears(which asks for username and password) when the user gives the username and password it does not recognize the user.What would be the problem?But the same user can access the folder which is inside the test machine(where project is launched) and he can also ping with the test machine.
The problem is its not recognizing the user.Hope anybody can solve my problem.
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