trouble resolving names in GOV.

We have two servers running BIND - one is running v9.2.1, the other is
running v9.3.0. We're seeing problems resolving names in the GOV tld on
the v9.3.0 server, and I can't see if this a problem in BIND v9.3.0 or
something wrong in the GOV. domain, or (most likely) something daft that
I've done. It looks as though the v9.3.0 nameserver is unable to resolve
anything in "GOV."

I do see differences in response if I use "dig" to try and find
nameservers in GOV compared to nameservers in other TLDs, for example,
compare looking up the namservers for "microsoft.com." and "nih.gov.":

1a) find the nameservers for "com."
$dig ns com.

; <<>> DiG 9.3.0 <<>> ns com.
;; global options: printcmd
com. 172800 IN NS k.gtld-servers.net.
com. 172800 IN NS e.gtld-servers.net.
com. 172800 IN NS m.gtld-servers.net.
com. 172800 IN NS a.gtld-servers.net.
com. 172800 IN NS g.gtld-servers.net.
com. 172800 IN NS h.gtld-servers.net.
com. 172800 IN NS c.gtld-servers.net.
com. 172800 IN NS i.gtld-servers.net.
com. 172800 IN NS b.gtld-servers.net.
com. 172800 IN NS d.gtld-servers.net.
com. 172800 IN NS l.gtld-servers.net.
com. 172800 IN NS f.gtld-servers.net.
com. 172800 IN NS j.gtld-servers.net.

1b) find "microsoft.com" nameservers:
$dig @a.gtld-servers.net. ns microsoft.com.=20

; <<>> DiG 9.3.0 <<>> @a.gtld-servers.net. ns
microsoft.com.
;; global options: printcmd
microsoft.com. 172800 IN NS ns1.msft.net.
microsoft.com. 172800 IN NS ns2.msft.net.
microsoft.com. 172800 IN NS ns3.msft.net.
microsoft.com. 172800 IN NS ns4.msft.net.
microsoft.com. 172800 IN NS ns5.msft.net.

This looks OK, but if I try the same for "nih.gov" (for example):

2a) find "gov" nameservers:
$dig ns gov.

; <<>> DiG 9.3.0 <<>> ns gov.
;; global options: printcmd
gov. 172800 IN NS c.gov.zoneedit.com.
gov. 172800 IN NS b.gov.zoneedit.com.
gov. 172800 IN NS a.gov.zoneedit.com.
gov. 172800 IN NS g.gov.zoneedit.com.
gov. 172800 IN NS f.gov.zoneedit.com.
gov. 172800 IN NS e.gov.zoneedit.com.
gov. 172800 IN NS d.gov.zoneedit.com.

2b) find "nih.gov" nameservers:
$dig @a.gov.zoneedit.com. ns nih.gov.

; <<>> DiG 9.3.0 <<>> @a.gov.zoneedit.com. ns nih.gov.
;; global options: printcmd

Shouldn't 2b return the list of nameservers for the domain "nih.gov"? If
not, why not?

Thanks in advance,

Steve Bennett