Each domain name is associated with two DNS servers, a primary server and a backup server. Once the ISP's computer finds the IP address it can access the web site. IP addresses are generally 12 digit numbers that identify a computer on the internet. IP addresses are like street addresses. They refer to specific servers and their physical location on the network.
Domain names are like the house or apartment addresses, they refer to the actual website.
There can be several reasons for this. This is out of our control and is done through your domain registrar. Also you ISP may have some lag time. Even though they are supposed to update every 72 hours it does sometimes take longer. As part of the application process InterNIC requires two domain name servers to link to your domain name.