Xiaomi
Add a Fingerprint auf Xiaomi
MIUI treats every fingerprint as a separate entry, so unlocking with your left hand only works if that hand's fingerprint was actually saved. Most people register one finger during the initial setup and never come back to add a second or third one, which is why the sensor sometimes refuses to react to the other hand.
Before you can add a new fingerprint, MIUI asks for your current unlock method as a security check. This confirms that the person adding biometric data is the actual device owner, not someone who just picked up an unlocked phone.
To add one, go to Settings ➔ Passwords & security ➔ Fingerprint unlock. Enter your current pattern, PIN or password when asked, then tap Add fingerprint. Enter the pattern, PIN or password again, then place the finger on the sensor and follow the on-screen instructions until the scan completes.
Once saved, you can rename each fingerprint from the same Fingerprint unlock screen, which makes it easier to tell entries apart if several people use the phone. If the sensor keeps failing to read a finger, cleaning the screen or sensor area usually helps more than re-registering it right away.