Why is my touch heart rate reading erratic or non-existent?

"Touch heart rate readings depend on a strong, clear pulse to be read from your hands. In order to generate a clear signal, gently grasp the sensors. Do not tightly squeeze them, as this will negatively affect the pulse in your hands, making it more difficult for the sensors to get a good reading.
The sensors will be more successful if your hands are warm and moist. If you have difficulty getting a reading right away, try again after a few minutes of working out. Typically exercise will cause most people’s hands to become warm and moist.
Touch heart rate is affected by an individual’s physiology. Some people have stronger pulse in their hands compared to others. Your results may vary."