When you change nameservers or DNS records, the change is not visible everywhere straight away. This is called propagation.
Internet providers cache DNS information for a period set by the record itself. Until that cache expires, some people will see the old details and others the new ones. This is normal and not a fault.
As a rough guide, most changes settle within a few hours, and you should allow up to 24 to 48 hours for it to complete everywhere.
During this window your site or email may appear to work intermittently. If it has been longer than 48 hours and things still are not right, open a ticket and include the domain name.
