In some instances, DNS servers may be uncontactable, or different DNS servers may give different results.
You can diagnose these types of issues by using a DNS traversal tool.
First convert your IP (e.g. 18.104.22.168), to the equivalent PTR format (in this case 22.214.171.124.in-addr.arpa). You can use a tool such as ‘dig x’ or http://whois.net/reverse-dns-ip-lookup/ to do this conversion for you.
Enter this PTR domain name (e.g. 126.96.36.199.in-addr.arpa) and type “PTR” into http://dns.squish.net. Complete the Captcha and start the traversal. It will take few minutes and show any problems it finds.