DHCP troubles

i have configured DHCP on Win 2000 for 2 of my remote subnets using relay agent & DHCP Superscopes. its working fine with both remote subnets. i have reserved an IP address for laptop Users on both subnets using the LAPTOPS’s MAC address. now when a users moves FROM REMOTE SUBNET A to REMOTE SUBNET B . he still gets the IP address of REMOTE SUBNET A which was binded to his laptop’s MAC Address. he should actually be getting the IP adderss from REMOTE SUBNET B when he is there in REMOTE SUBNET B. i have released & renewed IP assignment on his LAPTOP. but he is still getting his reserved IP address of REMOTE SUBNET A. how do i resolve this issue?