Windows 7: DHCP server issue with all windows sevens in wireless network

hi everyone,

I am challenging a problem in a network with windows seven,
I have wireless wifi area in which I am provideing wireless internet.
my DHCP server and Router is a win2k3 based system, which works fine for all clients, such as win xp, windows mobile, and every hand held I have tried there.



But my problem is with windows seven clients.
some of them doesn`t get IP from DHCP server.
and some get but cant ping the default gateway, it says destination host unreachable.
I have tried setting manual IP address on these clients but it doesn`t work and I get the same error.
I even replaced my WIN 2K3 with a linksys WRT to perform NAT and DHCP server, but I got the same results again.

I can not force my customers to change their windows from seven to any thing else.
this issue is putting me in a bad situation in my job,
please help me.