When viewing the Monitor Site Servers Status web part, the number of resources displayed is less than what is expected. For example, there is an agentless Monitor Policy which is applied to 200 resources, but this web part only displays 100 resources as being monitored.
Furthermore, agentless data is not available on some of the resources that the agentless policy is applied to.
This behavior likely occurs based on the Sites and Subnets configuration. Each site requires a Site Server, with the Monitor Service installed (Remote Monitoring Server, or RMS), applied to it in order for all resources to be monitored properly.
Ensure that each site and subnet has a Site Server, with the Monitor Service installed, assigned to it. A Site Server can have additional sites, beyond the default assignment, if necessary.
The following article provides more detail on how Site Server configuration directly affects agentless monitoring: