Renault Scenic heater/blower direction flap stopped working

The heater and blower work very well, provided you only want the flow to the cabin. If I move the knob to allow the air to go up the windscreen or down to the feet I get diddly squat in that direction. It is as if the flap (if that's what controls it) has either broken or come adrift. Any one else had the same problem and how is it fixed? Many thanks.