There is a heartbeat going on between the RHQ agents and the server: the agents are periodically reporting the availability of their managed resources to the server. Now if the server doesn't get those reports for some time, it will report the managed resource as being down. This might have three causes
- The resource is really down
- The agent is down
- The network between agent and server is down
- Combination of the above
So if a resource is down and you are not sure, go to its inventory tab and scroll to the bottom. There is a section "Agent Managing this Resource":
This shows the availability of the agent and when the last availability report for this resource was received.
If this one shows red, you really should have a look at the machine hosting the agent and the resource.