After investigating the issue it was determined that the customer was exceeding the maximum number of threads in the threadpool. This would create a situation where the threadpool would be filled with threads blocking while waiting for a callback. This callback most likely required room in the threadpool and therefore was unable to unblock any of the waiting threads. This situation is resolved by "tuning" the threadpool or manually setting the maximum number of allowed threads via ThreadPool.SetMaxThreads to a value that is greater than the number of requests the customer is expecting to handle.