Dust and dirt can collect over time on the optical sensor that reads the movements of the rollerbar. This can cause the cursor to navigate poorly. Most related problems can be rectified by cleaning around the sensor. To do this, apply compressed air from a can at and around the sensor.
The position of the sensor is shown here:
RollerMouse Free2- The sensor is located inside the rollerbar channel, under the plastic wall on the side nearest to the upper-left button.
(Photo of Free2 sensor location)
RollerMouse Pro2 and Classic2- The sensor is located 3 inches to the right of the paste button, under both the plastic and the rollerbar (Under dim lighting you can see the red light shining under the plastic when your RollerMouse is connected.)
(Photo of Pro2 sensor location)
RollerMouse Red- The sensor is located inside the rollerbar, protected mostly from dust and dirt. (Please refer to the "How can I remove my roller bar to clean?" article under the RM: Red Troubleshooting section.