Unmanaged monitoring service discovered but targets never becomes active and are dropped

When configuring un managed monitoring against the user monitoring Prometheus instance as per the documentation [1]
Prometheus service discovery discovers the labels but no active targets are found 

when the operator is installed in "all namespace" mode in the same cluster the monitoring is managed and configured successfully against the cluster monitoring Prometheus instance. 

The following documentation has been used to troubleshoot this issue confirming labels match in both the service and service monitor. 
Debug has been enabled within the user monitoring prometheus instance, I see the service being discovered but nothing to indicate why the service is dropped other than no active targets are found 

[1] - https://access.redhat.com/documentation/en-us/red_hat_quay/3.6/html/deploy_red_hat_quay_on_openshift_with_the_quay_operator/advanced_concepts#monitoring-single-namespace
[2] - https://docs.openshift.com/container-platform/4.9/monitoring/troubleshooting-monitoring-issues.html