Double check your firewall/routing rules also. A couple times when I had issues with certain zones, I caught IP's being blocked by the firewall. Set up the port forwarding rules, or reverse-lookup the IP's getting blocked in your firewall logs and if you find ones for POL or registered to SE, whitelist them (some don't return DNS names, but if you research the IP somewhere like dnsstuff.com, you will see that it is querying the SE DNS servers for info).