The DC11 has an unusual bypass turbine arrangement. The dirt does not go through the turbine. Instead, the dirty airflow goes straight up the tubes.
The air that drives the turbine, comes in via the net filter on the left-hand side of the head. Check that this is not clogged with fluff, hair or debris, by running the hose itself (with or without dusting brush) over this net filter.
Be sure that this filter-housing is pulled outwards from the head (this filter-housing is also the 'Brush ON/OFF' function).
Remove the turbine head from the extension tubes/wand.
Remove its base plate on the underside.
Remove the brushrolls (they twist off).
Use a pair of nail scissors to cut off threads and long hair, from the brushrolls.
Use the cleaner hose/crevice tool to suck dust and debris from the inside of the brushrolls.
Take a tissue or cloth over the plastic surround of the metal drive spindles, to remove dirt and grit.
Check that there is no fluff or debris blocking the soft rubber shield, in the dirty airflow path (from the brush cavity).
Check the airflow path at the join of the turbine-head to the wand/tube. There might be something blocking it.
