I had an issue lately where a new subdomain I’d added for a site wasn’t accessible. Trying to debug it, when I ran nslookup sub.example.com my.webhost.dns it returned the correct IP address of the server where the subdomain was meant to be pointing. But when I ran nslookup sub.example.com 8.8.8.8 (that’s Google’s DNS server) then it couldn’t find the domain.
Eventually I tracked down the problem, and it was something very simple. The domain wasn’t actually set to use my webhost’s DNS servers. Instead I had it configured to use CloudFlare’s DNS servers.
So if you have this problem, make sure you double-check that the DNS server(s) you’re updating are actually set as the primary DNS servers for the domain. It might seem obvious, but it’s easy to overlook (at least it was for me!)