1. REMOVE/DELETE all of the existing FINGERPRINTS.
2. RESTART/REBOOT the phone.
3. Your fingers should not be Harsh/HARD DRY while registering fingerprints, as it's WINTER season the Skins become hard & dry that may cause the issue.. It should not be wet too, just a normal moisturized finger should work.
4. Screen protector/Tempered glass also can lead to this issue.
5. And finnally, the main couse can be your Ambient lighting. [This issue I faced personally] : If your phone has Optical Fingerprint scanner/sensor then whenever Direct light falls on the screen.. at that time Fingerprint scanner doesn't work as it should.. It's working capabilities may reduce due to that direct lighting...
Solution- TRY REGISTERING YOUR FINGERPRINT IN A DARK ROOM /IN A ROOM WITHOUT DIRECT LIGHTING TO THE PHONE FINGERPRINT SENSORS.
