Subdomain delegation only returns SOA on dig

[Prabhat Rana]

Hello all,
I'm running BIND 9.6.1-P1 on a Solaris box. This DNS (ns1.spx.net) is authoritative to domain spx.net (this is just example). And I'm trying to delegate nse.spx.net to ns1.nse.spx.net. I think I have configured correctly but when I run a dig from a different DNS node for a subdoamin within nse.spx.net like mil.nse.spx.net, it responds only SOA in the Auth section. Its missing the NS from the zone files. The snapshot of my named.conf file

zone "spx.net" {
        type master;
        file "/opt/named/db.spx.net";
};

zone "nse.spx.net" {
        type master;
        file "/opt/named/db.nse.spx.net";
};


Here are the snapshot of consecutive zone files
$ttl 38400
spx.net.     IN      SOA     ns1.spx.net. ns2.spx.net. (
                        1189784076
                        86400
                        3600
                        604800
                        38400 )
spx.net.     IN      NS      ns1
spx.net.     IN      NS      ns2
ns2.spxdns.net. IN      A       10.1.2.3
ns1.spxdns.net. IN      A       10.4.5.6
ns1.nse.spx.net. IN        A    10.7.8.9
;there are other entries here
$ORIGIN nse.spx.net.
@      IN      NS      ns1.nse.spx.net.


And the 2nd zone file for submdomain nse.spx.net
$TTL 3600       ; 1 hour
@                       IN SOA  ns1.nse.spx.net <email> (
                                2008081812 ; serial
                                1800       ; refresh (30 minutes)
                                900        ; retry (15 minutes)
                                604800     ; expire (1 week)
                                3600       ; minimum (1 hour)
                                )
;
nse.spx.net.         IN      NS      ns1.nse.spx.net.
ns1.nse.spx.net.   IN      A       10.25.130.75


Now when I run a dig for say mml.nse.spx.net I get only the SOA of the above zone file and no NS information that the query is being delegated to.
#dig @ns1.spx.net mil.nse.spx.net
; <<>> DiG 9.6.1-P1 <<>> @ns1.spx.net mil.nse.spxdns.net
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 1717
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;mil.nse.spxdns.net.            IN      A

;; AUTHORITY SECTION:
nse.spx.net.         3600    IN      SOA  ns1.nse.spx.net <email>   . 2008081812 1800 900 604800 3600

;; Query time: 3 msec
;; SERVER: ns1.spx.net#53(10.1.2.3)
;; WHEN: Mon Mar 29 19:26:45 2010
;; MSG SIZE  rcvd: 108

How would the querying DNS find out about the nameserver that this subdomain is being delegated to? Why the query answer doesn't include NS sections. I've tried to change few things but nothing works. The only information I get is SOA and no NS in the AUTHORITY SECTION.

Any help would be much appreciated.

Thanks
Prabhat.