Need Help Regarding private nameserver

I am trying to set private nameserver for my domain. For this, I have entered ns1 and ns2 entries in the simple zone editor under cpanel. Also I have added Host entry in godaddy account and updated my nameserver there. So now if I check whois information, correct information is being displayed.
( I mean ns1.domainname.com and ns2.domainname.com are being displayed as nameserver )

But when I do DNS lookup under site like network-tools.com, there it will show the original server and not my private nameserver. For example:

network-tools.com/default.asp?prog=dnsrec&host=agro-lanka.org
under the "server", it is showing "ns1.ndx20.com" which is the original server. So I am not able to hide the original nameserver( this is just an example, not my site, but similar thing is happening to me )

But if I check for others' website which has set private server, there own private nameserver is being shown under "server" entry.

For example:
network-tools.com/default.asp?prog=dnsrec&host=cobithost.net
here you will see that it is showing "cobithost" under "server"

So can anybody tell me what is the problem in the my case?

It usually takes a day or two for third party sites to show the changes.

Did you register the nameservers with IP address at the Registrar the domain is hosted at? Each nameserver needs to be registered also at the Registrar.

I should of looked at your links before commenting. You need to edit the DNS for your domain and replace the nameservers ns1.ndx20.com with ns1.agro-lanka.org and same with ns2. Your DNS entries still show the old nameservers. Don't forget the dot at the end of the nameservers when you enter them.

[TABLE="width: 100%"]
[TR]
[TD]agro-lanka.org[/TD]
[TD][/TD]
[TD]NS [/TD]
[TD]ns1.ndx20.com[/TD]
[TD="align: right"]86400s[/TD]
[/TR]
[TR]
[TD]agro-lanka.org[/TD]
[TD][/TD]
[TD]NS [/TD]
[TD]ns2.ndx20.com[/TD]
[TD="align: right"]86400s[/TD]
[/TR]
[/TABLE]

Also I like intodns.com for this sort of work. Here it clearly shows the issue:

http://www.intodns.com/agro-lanka.org