Hi,
please check how many licences are being used or are available to systemhound. This is on the front-page of the systemhound web-site on your systemhound server. E.g.
Licensed to : Software Innovations UK Ltd
License : 3029 of 5000 used
Also check the log-file on some of the machines that have the clients on that have not reported to the server. The log-files will be situated in
C:\Program Files\systemhound\logs
See if there are any errors regarding contacting the server (in upload.log). It could be a DNS issue for example..