When this occurs I typically restart the McAfee RSD Sensor service and the state changes to Active or Passive again. It would be really handy to have an Automatic Response option to restart the service when a sensor changes to a 'Missing' state.
Perhaps, take a look at server settings / rogue system sensor: sensor per subnet and active period - may need to be adjusted. Also, look under rogue system detection policy / general: active sensor election - may need to be adjusted.
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