Please check if these other missing systems are actually part of the same subnet as the Sensor. The Sensor may not be detecting traffic if the broadcast traffic from other LAN segments is not forwarded across.
It is recommended that you install atleast 2 sensors per LAN segment (subnet) to cover all the machines in different segments.
If these are all part of the same subnet, you can examine if they are being detected by the sensor by looking at the RSD Sensor debug log.
The log is at Program Files\McAfee\RSDSensor folder named RSDSensor_out.log. You will need to enable debug on this log by editing RSSensor_out.cfg file and modfiying the line: