We have downloaded a trial of the NetMaster program to track data usage in our company. Perhaps this is just a limitation of the free trial edition, but each computer only shows a maximum of 2 computers in the dropdown box.
The free trial does not restrict how many computers you can connect to. The trial if fully-functional and is only limited in its duration (30 days).
Common reasons for the absent computes might be:
Other machines do not have the NetMaster system service installed. You need to install the service on all machines that you want to manage. Then they will announce their presence on the network.
A firewall may be blocking the NetMaster announcements.
Machines may be in a different subnet, in which case the broadcast announcements will not work.
If the NetMaster System Service is installed on each machine that you want to manage, please try connecting to the machines that are missing in the drop-down list directly by their name or IP address. This should work for the machines in a different subnet, and in some cases through firewall.