More then likely, they have the ip address cashed in their host file ( on there system ).
Or there provider has not been updated yet.
Is this DNS server from your provider or are you using one of the free DNS forwarding services.
Free forwarding services are not very reliable thus they go down all the time and can never be counted on.
You may want to go with a paid service. Some are not expensive, like 40-50 per year. I will try and did up some links.
It's been a while since I have used any DNS service other then the one that comes with my web service provider.
Check out this page for dns providers
http://dns.highsynth.com/
I have some better sits just need to find them.
Good luck