In the Brush Settings panel (F5), ensure that under Shape Dynamics > Size Jitter > Control is set to Pen Pressure. Under Transfer (Opacity Jitter), set Control to Pen Pressure.
Do not set the Flow to 100% and forget it. If you paint at 100% flow with max pressure, you lose the "dry brush" effect. Keep your Flow between 20% and 50% and let your pen pressure do the work. nikko rull brush for photoshop
The default Nikko Rull already has a secondary texture (usually a fine grain). Do not remove this. That grain is what prevents your art from looking like plastic. Nikko Rull Brush for Photoshop 1