So today I was trying to resolve an issue with a domain we host. Two of our name servers were sending the correct info, the third was not. In the logs of the third name server I was seeing entries like this:
Mar 1 08:26:22 ns3 pdns: Not authoritative for <domain> sending servfail to <ip>
It turns out that this is caused by missing SOA records for the domain. In fact in my database there were NO records for this domain. I added the appropriate records in and it now serves!