From this forum thread: http://www.jppf.org/forums/index.php/topic,1240.0.html
The threads includes a sample that reproduces the issue.
Looking at the driver and node logs, I can see that only a few out of 20 tasks are actually submitted. This indicates that the problem is on the client side.
Furthermore, if I enable debug logging on the client, or add a small delay after each task submission, the problem disappears. This also indicates a synchronization problem in the executor service.