Monitor's "Response Time-out” Setting is low

To resolve this issue, raise the Monitor's "Response Time-out" setting. Also ensure that the "Interval" setting has to be more than the "Response Time-out" setting. For example, you cannot have a "Response Time-out" setting of "8" seconds and an "Interval" setting of "5" seconds. For detailed information about Citrix ADC Monitor Parameters refer to article CTX132937 - Citrix NetScaler Monitor Parameters: Retries, FailureRetries and Downtime

The monitor probes are low level probes and sometimes a server might ignore them at first because of Higher Priority services it has to respond to. When it has time, the server will reply to the probe. Increasing the response time on the Monitor, keeps the Monitor UP until it receives a reply to the probe.

Firewall Misconfiguration

Ensure that Firewall in not blocking the SNIP from connecting to the backend server.

Backend server either not listening or unreachable

Ensure that the backend server is reachable and there are no network issues.