Fingerprint ridge-detail can be “damaged” - and my result in your registered fingerprint not be recognised. Try re-registering your fingerprint(s):
Use Touch ID on iPhone and iPad - Apple Support
If TouchID continues to misbehave:
If Touch ID isn't working on your iPhone or iPad - Apple Support
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.