1. Ensure the DNS server service is running and it is healthy.
2. Check that the correct IP addresses of DNS servers are configured on the client machines.
3. Validate that the domain name resolution in the server is functional, by using dig, nslookup, and other tools.
4. Ensure DNS replication is in place between DNS Servers.
5. Make sure DNS records registered in the zone are correctly listed in the DNS server.
6. Also check for proper DNS zone transfer settings.
7. Validate that the DNS resolver cache is enabled and can be used properly.