DHCP Server not assigning IP address after ISA 2004 installed

Hi

We are process of upgrading from SBS 2000 to WIndows 2003 Domain. Domain controller upgrade is successful. We were tesed computers and user logins attaching to new domain controller(Windows 2003 Domain). Was working fine. We installed ISA 2004 and configured rules for internal and external network. I created rule for DHCP as mentioned in the article http://www.microsoft.com/technet/prodtechnol/isa/2004/plan/isaondhcpserver.mspx . Still Clients not able to get IP address from DHCP Server. If you have expreience the same problem, please send me the information how you fixed the problem. I really appreciate that.
Thanks