no lateral growth on wisteria

I have a white four year old wisteria which has never flowered.  I have grown it up a 10ft wall intending to train the side shoots along lateral wires.  However, from a height of about 3ft to where it has reached the top of the wall there is no more lateral growth at all, just a bare central stem.  The top is full of new growth which is now growing forwards away from the wall where it is supporting a clematis. Now the clematis has finished flowering I am tempted to lop the top of the wisteria off to encourage lateral growth further down.  Is this a good idea or total madness??