In some cases, the state of the finger itself can lead to lack of recognition. The fingerprint scanner only works correctly and reliably with the papillary lines of the front phalanx. If your finger is very dry, wet, or cold, these papillary lines cannot be detected sufficiently.
If your finger is very dry:
If your hands are dry, it often helps to run your finger over your forehead.
If your finger or the sensor is wet:
Dry your finger.
Dry the sensor of the fingerprint scanner with a non-abrasive cloth.