The RPC Server is unavailable indicates that the CCS Manager (for CCS 11) or the Query Engine (CCS 10.x) was not able to connect to the target machine when it tried to process it for the particular query or data collection job.   A couple of things could be causing this message:

1. The target machine is down at the time when the query is being processed
2. The job was distributed to the wrong Manager/Query Engine
3. for CCS 11  Check the routing rules
4. for CCS 10.x  Check the distribution rules.for the Master Query Engine.
5. Also verify that there are no firewalls between the CCS Manager/Slaver Query Engine and the target that might be preventing connectivity.

Thanks for your reply. Among all 5 items only the 2 may be the reason. I suppose we have to check the routing rules? don't have any firewall blocking and target machines are not down but just in case, which ports should be checked on the targets?

Ports on Windows machines should be:  135, 137-139, 445  initiated from CCS Manager Machine to the Target machine.  Look at Page 46 in the CCS Planning and Deployment Guide.