The necessary tables that are used for the Computer Summary report only gets updated when the NS.Daily task runs during the night. If you are missing a large portion of systems in this report its either that they just reported their inventory and the daily hasn't ran yet or the process is timing out.
To confirm if the process is timing out you can try these simple steps on the NS.
1. Open the NS Log Viewer.
2. In Scheduled Tasks start NS.Daily
3. In the Find field of Log Viewer enter 'generating'.
You will see 5 different informational log entries pertaining to generating the summary data class. At the beginning of each data class generation the log entry will be created. If it takes more then 10 minutes for the next generating log entry to appear then the process has timed out.
If this is the case please contact support.