1) Check if the sensor is turned on. Make sure that the pen has working batteries.
2) Check if you see a red light from the pen tip when it is pressed. (It is harmful to your eyes to look directly into the red light. Try to check in a dark place.)
3) Check if you see a red light on the LED indicator of the sensor when the pen tip is pressed.
4) If you find no problem in checking the 3 items mentioned above, you will need to relocate the sensor.