All tasks, in what context? That are assigned to you? Or all tasks for a specific process instance? The task service is implemented so that you can only get the tasks that are relevant to you. So that means you have to be assigned as a potential owner or similar to be able to "see" the task.
It would be possible to simply retrieve all tasks as "Administrator" as this user is always added as the administrative owner of a task in jBPM5, so that should allow you to get all tasks.