You will need to place the finger in the correct position for the phone to recognize you as an authorized user. Upon authorizing the Finger Print/Touch ID, you will need to ensure that you remember your expression and position of the camera on your face when registering to login successfully.

The best practice is, if using facial capture as a biometric that this is refreshed as a maximum every 5 years, however, the recommendation is to re-register Biometrics every 3 years.