Domain propagation can take anywhere from 24 to 72 hours normally after you register a domain is registered or change the nameservers. Before propagation has completed it is not unusual to be able to reach domain.com and not
www.domain.com. This happens because while some DNS servers get updated, some don't. Give it about 72 hours and if it still doesn't work, then submit a trouble ticket to the registrar. If you've used WHM/cPanel and configured access to domain.com, then once propagation is complete, it will automatically add and recognize the World Wide Web prefix.