nslookup debug

Try adding forwarders to some public DNS servers leave the box ticked which says use root hints if forwarders are not available. Also like Jermey said if you run the following you can look at the debug information of the DNS lookups

NSLOOKUP

SET DEBUG=ON

domainname.com

That will show you what is happening with the request you can also use SET D2=ON which will give you even more information on the lookup.