reverse problem

[bind-list]

hiya, i'm fairly new to the list and i was wondering if anyone could help me 
with a problem i'm having with reverse lookups for a c-class

our nameservers are ns1.pwebtech.com and ns2.pwebtech.com and the block is 
64.21.139

for some reason the block just will not reverse and i've checked everything i 
can think of. all our files appear correct, and our upstream (ns1.nac.net) 
assures us that all is delegated correctly

any ideas?

thanks in advance, 
Dave