are you able to ping the host?
under the search option you can set it to search even if ping fails
from a previous post :when i clear the check box of ( continue searching even if the icmp ping falis ) from the search for unmanaged computers options i rarly have this error but i don't this is a good solution"
or
enable smc loggin to finest lets see why its failing
http://www.symantec.com/business/support/index?page=content&id=TECH104804&locale=en_US