When Touch ID is not working on your iPhone/iPad, the most possible causes can be: 1. User Errors To make Touch ID work properly, you need to place your finger not only on the home button, but also on the stainless steel ring. Meanwhile, since the capacitive sensor is made of raw silicon, it is sensitive and fragile to circumstances, like moisture or dust. The recognition will fail if your hands are too oily or dry (especially in winter). 2. Touch Sensor Although the capacitive sensor is covered with sapphire crystal, a hard material, it will cease to work properly when the home button gets scraped or scuffed. So as the metal ring surrounds it. You may fail to use Touch ID if the crystal layer or the metal ring gets damaged.