Catching QoS messages from sink on pipeline bus

[stic at free.fr]

Hi,

I am trying to catch qos message sent on the pipeline bus but I am unable to get them.
Basically I am setting the sink with qos=true parameter to enable qos messages from the sink, I instruct the bus to emit signals for received messages, and I connect the signal to a callback function by using following code: g_signal_connect (G_OBJECT (bus), "message::qos", (GCallback)qos_cb, data);

I am already able to catch any other message such message::state-changed or message::error, etc... but the qos messages are never sent, the qos callback is never called.

Is there something I am missing here ? 
What I would like to achieve is adjusting the upstream bitrate when there is too much network usage.

Any help would be really appreciate.
Thank you very much.