My security light sometimes comes on and sometimes doesn't

--The motion sensor on your light is not an infared sensor that many AC security lights use and instead activates when motion is detected within the sensors field of vision for a certain amount of time. The reason why something has to be moving for a certain amount of time within the motion sensors field of vision is so that it can't be tricked by lets say a leaf flying by or a bug flying past. So moving in then out of the motion sensors field of vision in a prompt manner will not activate it to turn on the light. The closer you are to the light the smaller the width of the motion sensors field of view is thus making it much faster to move in then out of its vision. The other problem is that this field of view is only within a certain amount of feet from the sensor meaning that if your to far away it will not trigger it. you will have to adjust somewhat to a happy medium between distance and the sensitivity knob (If equipped with one).