You're not alone in having difficulty using TouchID. I too have seasonal issues with TouchID devices, usually during winter, when humidly is low and skin is particularly dry.
TouchID is usually reliable with regular use (the sensor updates its biometric map with every successful fingerprint authentication), however, prevailing conditions can be problematic for a completely healthy TouchID sensor.
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Some users just have difficulty using TouchID. The causes for of TouchID being unreliable for some users - and not others - are many. Whilst not an exhaustive list, possibilities include:
- Very high humidity (“sweaty” fingers)
- Very low humidity (excessively dry skin)
- Poor fingerprint ridge-detail (excessive hand washing - or exposure to abrasive material, damaging fingerprint detail)
- Physical injury (cuts or splits to the fingertip)
If you have naturally dry skin, try using a “light” (non greasy) and easily absorbed skin moisturiser.