.1. Firewall software. Usually, a significant factor for resolver failures will be firewall program constraints. By default, the majority of servers enable relied on IP address in the firewall program. All DNS inquiries will stop working if for any factor the connection server IP is not white-listed. And, the DNS resolvers ended up being inaccessible.

.2. Timeout mistakes. Similarly, the connection to the DNS resolver can even time out too. The factor for timeout can be network mistakes, ISP limitations and a lot more.

In this case, the secondary and main DNS resolvers were revealing timeout which activated the Partial DNS resolver failure notice.