NT Client DHCP problems moving between subnets

I’ve got an NT Workstation client that has problems moving between subnet 1 and subnet 2. (Same NT domain)

For example, he normally resides in subnet 1, but on occasion travels to subnet 2.

When he arrives at subnet 2’s location, he is not able to get an IP address from the DHCP server which is located in subnet 3. Router in subnet 2 servers as DHCP relay agent. The only way to get him to obtain IP address from subnet 2 is to do a manual IP release and renew.

Any idea why the client PC is not releasing the IP address when he shuts down in subnet 1?

Thanks.