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Are you using a screen protector? unofficial, scratched or dirty screen protectors may cause the fingerprint sensor to malfunction. Try removing or cleaning the screen protector
If your fingerprint still isn't being recognised, you can try removing and then registering your fingerprint again.
Open the Settings app-Tap Biometrics and Security-Tap Fingerprints-Enter your PIN, pattern or password-Tap the registered fingerprint-Tap Remove-Tap Remove again to confirm Tap Add fingerprint
Follow the on-screen instructions to register your fingerprint again
If your fingerprint sensor is still having issues, you can try clearing the temporary data on your phone to fix the problem. This will not delete any of your information from your device.
Turn off your device
Press and hold the volume up button and the Bixby button or side button. Then press and hold the power key as well.
When the Android logo appears on the screen, release all three keys
Press the volume down button to select wipe cache partition
Press the power button
Press the volume down button to select yes
Press the power button again
Your cache will now be wiped. Once finished, Reboot system now will be selected
Press the power button to restart your device