The upper and lower media sensors must be aligned to properly function. The lower media sensor emits infrared light (invisible to the human eye) and the upper sensor acts like an eye receiving the infrared light seen between the labels. If you are using a hole or notch for top of form sensing the upper sensor must be positioned directly over the opening to see the light source.
Note: The black mark sensor is located under the printhead and is viewed as a red visible light when you open the printhead. This sensor is fixed in position and cannot be moved.