The TouchID sensor is an electro-optical component. The glass optical surface is surrounded by a thin metal ring; your finger must touch both the glass and the metal ring for a finger to be recognised.
Additionally, the optical surface of the top-button, that incorporates the TouchID sensor, has a relatively small surface area - unlike older models of iPad that had the sensor incorporated within the larger Home Button. This smaller area obviously requires good contact with your finger to read the ridge-detail of your fingerprint.
Some poorly designed cases can obstruct effective placement of your finger on the sensor. As such, arguably, it is the case design that is potentially at fault - and not, necessarily, a fault of the iPad. Consumers continually seek thinner devices that have smaller screen bezels, this introducing physical challenges in incorporating sensors and physical buttons that are sufficiently large to be effectively operated by the user.
FaceID offers substantial benefit as an authentication technology - in that it doesn't require physical contact. FaceID is available with iPad Pro models.
