I get this all the time from the "start-up SEOs" they go to the trouble of using all manner of nameservers rather than our provided ones, then cannot understand that they must resolve the www/website address to the same IP that they hosted their domain on (we give a choice of IPs).
It's a nightmare explaining that if they change the domain IP, they have to change the DNS.
"But that should be automatic" the say.
It would be if they used our nameserver cluster.
Usually I provide them with a special checklist, which if they follow, will ensure their domain is working.