"Versions of Windows NT DHCP server prior to Windows NT 4.0 Service Pack 2 are not capable of assigning addresses from more than one scope to a given physical subnet. One workaround for this situation is to add additional network interface cards (NICs) to the server, and to address each of the NICs to a given logical IP subnet. This involves additional and otherwise unnecessary hardware, so a new solution was developed and implemented in SP2."
(from MS KB Q161571)
get the whole article - looks like it's your case.
Hope it'll help.