The short answer: Your authoritative name servers are out of sync.

wn.net  NS      wn1.wn.net
wn.net  NS      wn2.wn.net
wn.net  NS      nic.cent.net
wn.net  NS      ns2.cent.net

WN1.wn.net has no knowledge of wn6.wn.net, but the other three NSes do.

/Pete