All computers show a grey icon for the health status and they show up as Untracked on vac_agenthealth.
This can happen if the AC_AgentHealthSettings table is empty. You can check for this condition by running the following query.
select * from AC_AgentHealthSettings
If this returns no rows, then your agent health status will not work correctly.
You can resolve this conditiion by running the following SQL:
insert into AC_AgentHealthSettings
This will populate the default values in that table and allow your agents health data to work correctly. As Always, make sure you have a current backup of your database before making any changes directly.