Why my slaves in swarm are always in 'Busy'?

Hi,
I connected several PC(5or6) as slave for use in lighting build via swarm agent.
All the connections are seem to be no problem.
But their state are ‘Available’ very rarely in Swarm Coordinator.
Most of the time their status is ‘busy’, even though the computers are not doing anything.
Only one or two slaves can be used in each lighting build of the five or six slaves.
What determines the status of those computers?
How can I change the status of those computers to ‘Available’??