Added FAQ: Why isn't my tasks processed.

FAQ

@@ -2,6 +2,9 @@
  Frequently Asked Questions
 ============================
 
+Troubleshooting
+===============
+
 MySQL is throwing deadlock errors, what can I do?
 -------------------------------------------------
 
@@ -47,6 +50,38 @@ I'm having ``IntegrityError: Duplicate Key`` errors. Why?
 **Answer:** See `MySQL is throwing deadlock errors, what can I do?`_.
 Thanks to howsthedotcom.
 
+Why isn't my tasks processed?
+-----------------------------
+**Answer:** With RabbitMQ you can see how many consumers are currently
+receiving tasks by running the following command::
+
+    $ rabbitmqctl list_queues -p <myvhost> name messages consumers
+    Listing queues ...
+    celery     2891    2
+
+This shows that there's 2891 messages waiting to be processed in the task
+queue, and there are two consumers processing them.
+
+One reason that the queue is never emptied could be that you have a stale
+celery process taking the messages hostage. This could happen if celeryd
+wasn't properly shut down.
+
+When a message is recieved by a worker the broker waits for it to be
+acknowledged before marking the message as processed. The broker will not
+re-send that message to another consumer until the consumer is shutdown
+properly.
+
+If you hit this problem you have to kill all workers manually and restart
+them::
+
+    ps auxww | grep celeryd | awk '{print $2}' | xargs kill
+
+You might have to wait a while until all workers has finished the work they're
+doing, if it's still hanging after a long time you can kill them by force
+with::
+
+    ps auxww | grep celeryd | awk '{print $2}' | xargs kill -9
+
 Why won't my Task run?
 ----------------------
 
@@ -96,6 +131,9 @@ same worker when it has been restarted), so to properly purge the queue of
 waiting tasks you have to stop all the workers, and then discard the tasks
 using ``discard_all``.
 
+Brokers
+=======
+
 Can I use celery with ActiveMQ/STOMP?
 -------------------------------------
 
@@ -173,6 +211,9 @@ using the STOMP backend:
 
     * mandatory
 
+Features
+========
+
 Can I send some tasks to only some servers?
 --------------------------------------------