What kind of ducting is there?
The wire re-enforced concertina type of ducting isn't the most efficient where the inside point of the "v" of the concertina sits in the duct lowers the diameter of it. Additionally it also creates turbulence in the air passing through it again reducing the efficiency. Finally water can collect in the hollows allowing stuff to grow and in worst case scenarios further block airflow.
Is the ducting in a straight line going direct from the fan unit to the vent?
You want to minimise the number of bends in any duct work. In solid ducting elbows have a radius to them rather than a straight 90 degree bend like you would see in pipework.
If the ducting is flexible could it have shifted for any reason since it was installed?