One computer in cluster not working

I have a cluster set up with three computers on the network - "This Computer", "Second iMac" (which is connected via Ethernet) and "Thunderbird" (a MacBook Air connected via WiFi). When I send compression batches to the cluster, only Second iMac takes on any of the load. Thunderbird doesn't. Why could this be?

When I look in Compressor > Preferences > Shared Computers, all the computers are listed and all ticked. But "This Computer" and "Second iMac" are marked as Idle (even when a job is running) yet "Thunderbird" is marked as Busy.

All help gratefully received.

Martin S Taylor