If you open a terminal and run ifconfig and see no IPv4 address in there then the interface has dropped the IPV4 and your ISP is marking the address as used, I dont think there is a lot you can do about that,
If still have that IP on your interface then I would recommend opening two terminal windows,
in one run a trace
In the other window do your dhclient command as that *should* work.
tcpdump -vvv -i any -s 1600 port bootps or port bootpc
Looking at the trace you will see if the IP is being relinquished.