Client doesn't get new IP when switching between subnets/locations

Client doesn't get new IP when switching between subnets/locations

So here is our setup.
We have 14 locations each with its own subnet and router. The routers at
these locations have DHCP relay enabled that points back to our central
server that is running DHCP.
This DHCP server (Windows 2008 R2) has a Superscope and then our 14 other
scopes inside of it. So far everything has been working and all the
locations have been getting the appropriate IP addresses.
However, when a user goes between locations they never receive a new IP
address. Their machine holds on to the same IP from the previous subnet
and the DHCP does not hand them a new one. It won't even work if we do a
release/renew.
I have to go into the DHCP Server and physically delete their IP entry in
order for them to get a new IP at the new location.
Any Ideas?