WSUS not detecting computers

Hi I have followed the online tutorial from Microsoft Technet and successfully installed an configured WSUS 3.0 SP2 on Windows Server 2008 x64. I also set up a new GPO for use with WSUS and set the required policies, however the WSUS service does not detect any computers in the Domain which obviously need updates. Can anyone help please? I have also whitelisted the server ip on port 80 (HTTP) and 443 (HTTPS) through Pfsense firewall. Thanks