If a client's IP address changes, most often when DHCP renews its lease to the client or a range of clients, the Remote Desktop admin application may not correctly acquire the new IP address for some clients. Remote Desktop identifies client computers using the hardware address, but has to use the last known IP address when trying to access the client. If the IP address has changed, you need to use the network scanners to locate the new IP address of the client.
To quickly resolve this issue:
If the client is found, its IP address will automatically be updated and your list of computers will again appear online.