Many DHCP clients have configuration switches that instruct the DHCP client not to use the nameserver(s) provided by the DHCP process. If you manually enter nameservers in /etc/resolv.conf, these are often overwritten by the DHCP process. So, be sure to check your DHCP configuration and make sure that your DHCP process is configured not to accept or request nameserver information from the DHCP server(s) if you want to use your own set of nameserver addresses.