Firewall for NFSServer under Suse 9.3

The NFS server appears to be blocked by the firewall under Suse 9.3. Having disabled the firewall, NFS server will serve quite happily. I then looked into the yast2 firewall settings, it does a. put all interfaces to be the "external" zone, b. NFS Client and Server are listed as allowed services for External zone. So why NFS server is not working with firewall enabled? Puzzled.