Why adding new user to Remote desktop group doesn't automatically creates him a folder in C:\users

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http://serverfault.com – I am administrating a remote Windows Server 2008 Standard. I have already 17 users in 'Remote Desktop Users' and all of them have home folders in c:\users. But when I've added the 18th user it didn't create a folder for him. Can you someone please suggest a reason? (HowTos)