This could occur when the lamp has not been lit in a long time or it is nearing the end of life. In these instances, our auto lighting circuit may not always catch and turn on the lamp. One means of testing whether the lamp is still functional, or whether the RF circuit is viable, is to light the lamp manually. To do so, turn on the unit like normal, and then remove the circular cap which holds the lamp in the sensor head. You will need to disconnect the suction line at the head, but the cable should otherwise remain connected. You can then slowly pull the lamp partially in and out of the holder until it begins to glow violet. If the lamp lights simply replace the cap and use the unit as normal. We can light the lamp in this manual way as we alter the electric field around the lamp when we pull it in and out. Because the action of altering the field by hand occurs far more slowly than with the automatic circuit, there is no virtually no risk of it not catching on if it is actually viable.