Hi Bill,
It’s possible that there is debris inside the laser counter (channel A) that is blocking particles from moving through or covering the lens. We recommend cleaning the sensor: Sensor Maintenance. As blowing air through the air channel can can the fan to move too quickly, causing damage, we recommend that you apply short bursts of air or use a toothpick to hold the fan in place.
If this doesn’t work, you can also replace the laser counters.