Hello,
are these windows machines?
do following command on both machines in cmd-promt:
ipconfig /all
There you should see the DHCP servers IP from which the client got an IP address.
An other thing that you should check:
what is the user session timeout on the controller?
what is your lease time on your DHCP server?
your user session timeout should be < 1/2 of the DHCP lease time.
Because if the session timeout is bigger: your client will idle/sleep and will keep the entry in the user-table (with the IP address), while on the DHCP server the lease time will expire and it will give out the IP Address again to an other client what will cause a conflict.
Kr,