Width Tool on a pen path

If you have a Brush applied to the Pen path the Width tool might not work. It works when certain brushes are applied to the path but  not with all of them. Select the path and check the Appearance panel or Brushes panel to see if a brush has been applied or to remove the brush.

Yes, you can use the Width tool on a Pen path. 

Could you please show the selected path together with the Appearance panel?