Please help with vectorfieldvisual3D

i have this:
HelixToolkit.Wpf.HelixViewport3D viewport = new HelixToolkit.Wpf.HelixViewport3D();
HelixToolkit.Wpf.DefaultLights light = new HelixToolkit.Wpf.DefaultLights();
viewport.Children.Add(light);
viewport.ShowCameraInfo = true;
this.grid.Children.Add(viewport);
Vector3DCollection vectors=new Vector3DCollection();
Point3DCollection points= new Point3DCollection();
HelixToolkit.Wpf.VectorFieldVisual3D vfield = new HelixToolkit.Wpf.VectorFieldVisual3D();

//then the calculations of the vector field
while(i<=niterx)
{
j=0;
while(j<=nitery)
{
k=0;
while(k<niterz)
{
//caculations of a vector function go here. xvalue,yvalue,zvalue are spaced on a grid, and the functions give vectorvalx,vectorvaly, etc
points.Add(new Point3D(xvalue[i], yvalue[j], zvalue[k]));
vectors.Add(new Vector3D(vectorvalx[i, j, k], vectorvaly[i, j, k], vectorvalz[i, j, k]));
k++;
}
j++;
}
i++;
}

vfield.Positions = points;
vfield.Directions = vectors;


vfield.Diameter = 5;
vfield.HeadLength = 2;
vfield.Fill = new SolidColorBrush(Colors.Black);


viewport.Children.Add(vfield);



i cant figure out why its not working. the vectorfieldvisual3d doesnt render. What am i missing?
please help and merry xmas