Customize External monitor not working to achieve service based failover

What does your monitor configuration look like. Are there more than one monitors assigned to the pool is so have you by any chance specified availability requirement. Make sure your monitor complies to https://support.f5.com/csp/article/K2167, also what version are you running on? You may have to take under consideration a known issue described here https://support.f5.com/csp/article/K77256432.