An execution server may no longer be recognized by the application server if the execution server's IP address has changed. Re-starting the application server means the execution server should be recognized again.
Java uses a cache to store the host name resolution to guard against DNS spoofing attacks. In Performance Manager the result of positive host name resolutions are cached forever, but this can be changed by editing the file java.security on the application server. This enables the application server to recognize execution servers even if their IP address has changed.
For more information on this Java setting, visit the Networking Properties page.