There are a number of restrictions the use of biometric identification which includes fingerprints and facial recognition. Assuming you are using 12.1…
  • check the settings in Settings > Face ID & Passcode. There you will find which apps are authorized to use biometric identification otherwise you will be required to enter your passcode
  • In Settings > General > Spftware Update if Automatic Updates are ON any iOSe update will reboot your iPhone and subsequent access will require you to enter your passcode to activate biometric identification
  • in Settings > General > Accessibiity if Guided Access
  • If your iPhone restarts for any reason you will have to enter our passcode to activate biometric identification
  • to change many Settings you must enter your passcode as biometric ID is not accepted
  • Not all apps accept biometric IDs — contact the developer and ask they add that feature and when it is added be sure and authorize its use in Settings > Face ID & Passcode
  • If for any reason your fingerprint is not recognized you will have to enter your passcode and although fingerprint ID is reliable there are situations when your fingerprint may not be recognized. In my experience facial ID is the more reliable but even then there are occasions when that fails. If there is any question iOS quite properly takes the conservative approach and require your passcode. If the failure happens often try erasing and resetting your fingerprints. Be particularly careful about the edges and tips of your fingers.

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