Every time you make a change to the DNS servers of your domain, or you modify any DNS record, you have to wait for the propagation of this data to be effective.
Propagation is the time it takes for each of the internet backbones to be updated with your new DNS information.
This often takes about 24 hours, but can take even longer, sometimes up to 5 days.
It's not linear: these data travel from one DNS server to another, and is influenced by the policy of every single server. So it may happen that a user see new DNS data in few minutes, while another user (with different connectivity and DNS server) take many hours.
You can check the propagation of your DNS worldwide using this free service: www.whatsmydns.net