Although your DHCP Server has been configured to assign a new IP address to a network machine;your DHCP client will retain its old IP address.
In such a case; clean out the Leases file on the DHCP Server and the DHCP Client side.
On Server side;
echo "" > /var/lib/dhcpd-*/dhcpd.leases
On Client side;
echo "" > /var/lib/dhclient/dhcp.eth0.leases