Little variation in stroke thickness

Line variations (thick and thin lines) are the basis of both Modern and Traditional Calligraphy which is hard to master without a lot of practice in order to build and control the stroke thickness (width).

Since the thickness of the line is dependant on the pressure applied on the down stroke (thick lines) and less pressure applied on the upstroke (thin lines) the flexability of the nib is important, as the stiffer the nib the less differance in thickness between the up and down strokes.

If the nib flexability is not the problem then you may need to change the way that you grip the pen. The standard tripod grip has your index finger on the side of the pen to guide and support the pens movement easily, whereas by changing your grip to the rotated tripod style you can exert more pressure and control on the nib through pressure from your index finger.