The problem might be that you’re using a custom brush. That’s one of the Ryan Kingslien brushes and was made with a much older version of ZBrush. You’re taking a chance using things that were created in older versions of ZBrush. I would suggest that you use one of the brushes that ships with ZBrush and alter it to do something similar. As an example that might match the RK_Ball Stylus brush is the Clay Tubes brush using Alpha 35 from the Alpha palette.