reverse zone delegation question

[dr sarcophagus]

hello.
i have a class-b block of ip addresses.
from that, i wish to delegate one or more /24-sized blocks of addresses to
another nameserver, but i'm having some problems getting this to work.

as i understand, since i'm using properly-aligned address blocks, i shouldn't
have to employ any rfc2317-style antics to get this to work.

in this, case, the zone file below serves 10.189.0.0/16, and i wish
to delegate administration of 10.189.21.0/24 to dns1.example2.com.

i've set up the 21.189.10.in-addr.arpa zone on dns1.example2.com, and can
query the SOA record for that zone on that server, but when I attempt to 
query the SOA record using the primary master here (dns1.example.com), it
fails.
queries of the PTR records below succeed.

this should be a simple case, and i believe my syntax to be correct.

any thoughts?  any help is much appreciated.


; --- 10.189.rev -----------------------------------------------------------
$TTL	6h
@			IN SOA	dns1.example.com. dns.example.com. (
				2006070612	21600	3600	3628800 21600)
;
;				Serial		Refresh Retry	Expire	Minimum
;				yyyymmddnn	6h	1h	42d	6h
;
@			IN NS		dns1.example.com.
			IN NS		dns2.example.com.
;
; 10.189.0.0/16
;
; reverse mappings to hosts
;
;
; 10.189.0.0/24 - Loopback Addresses
;
1.0			IN PTR	a001.example.com.
2.0			IN PTR	a002.example.com.
3.0			IN PTR	a003.example.com.
4.0			IN PTR	a004.example.com.
;
; 10.189.16.0/24 - Server Farm
;
1.16			IN PTR	b001.example.com.
2.16			IN PTR	b002.example.com.
3.16			IN PTR	b003.example.com.
;
; 10.189.21.0/24 - delegate
;
21.189.10.in-addr.arpa.	IN NS	dns1.example2.com.
;
; 10.189.128.0/24 - User Network 1
;
1.128			IN PTR	c001.example.com.
2.128			IN PTR	c002.example.com.
3.128			IN PTR	c003.example.com.