Adjusting the sensitivity would be something to try, but I doubt it will help in this instance. Many trackballs (and even some spinners) which have integrated interfaces, use an inexpensive "off the shelf" mouse encoder chip, which was never designed to be used in conjunction with these type of controls in this application. They are just too slow to deal with the rapid transitions they end up encountering.
The most likely reason it works when spun in reverse is that the ball is actually slipping on the rollers. They aren't designed to be spun fast in that direction. However, the slippage probably keeps the load lower on the decoder, so it doesn't miss steps / overflow.
The best thing to do with these types of units is to avoid them, unless you are using them only in low speed applications, like a mouse or slower games. By the time one realizes the shortcomings, it becomes very expensive to replace all the parts required to turn them into a standard unit.