reverse dns works but kinda doesnt (sub class c)

[Aj]

I have configured my ns as a primary for my 64 block of ip space, and i
have told my isp to make mine an authority for those 64 blocks.

so they did add all 64 cnames aliases to my nameserver. when i do
nslookup
ip address.. i do get the results back that i want but the thing is when
i go on IRC etc., with one of the hosts.. it doesnt reverse.. when you
do whois on it.. only the ip is showing. 

is this a problem in my configuratin or my isp's side? here is one of
the
ip address if you want to know more info, 209.157.120.36.

thanks for your help.