I have been programming a jogl application and I can't get deph testing to work. Can anyone with more experience than me sugfgest something i am missing, or something silly i have done?

I use the following code (in my class that implements GLEventListener) to enable depth testing,

      public void display(GLDrawable drawable)
      {
         GL gl = drawable.getGL();
         GLU glu = drawable.getGLU();


         camera.position(gl, glu);

         //enable removal of hidden faces
         gl.glClear(GL.GL_DEPTH_BUFFER_BIT);
         gl.glEnable(GL.GL_DEPTH_TEST);
         gl.glDepthFunc(GL.GL_LEQUAL);

         gl.glClear(GL.GL_COLOR_BUFFER_BIT);

         owner.display(drawable);

         gl.glFlush();
      }

where owner.display(drawable) calls a display object function in all the objects in the universe, each of which either draws a quad of a triangle, as follows:

      public void displayObject(GLDrawable drawable)
      {
         GL gl = drawable.getGL();

         if(fill) { gl.glPolygonMode( GL.GL_FRONT, GL.GL_FILL ); gl.glPolygonMode( GL.GL_BACK,  GL.GL_POINT); }
         else { gl.glPolygonMode( GL.GL_FRONT_AND_BACK, GL.GL_LINE ); }

         //gl.glCullFace(GL.GL_FRONT_AND_BACK);
         //l.glPolygonMode( GL.GL_FRONT_AND_BACK, GL.GL_FILL );


         gl.glColor4f(colour[0], colour[1], colour[2], colour[3]);

         gl.glBegin(GL.GL_POLYGON);

         for(int i = 0; i < points.length; i++)
         {
            gl.glNormal3d(vertexNormals.getX(), vertexNormals.getY(), vertexNormals.getZ()); //Set the normal for the node
            gl.glVertex3d(points.getX(), points.getY(), points.getZ());
         }
         gl.glEnd();
      }