1. Go to Settings > Touch ID & Passcode > Enter your Passcode. Then turn off the iTunes & App Store. Reboot your iPhone or iPad. Go back to Touch ID & Passcode in Settings and turn iTunes & App Store back on. Tap on Add a Fingerprint to add another fingerprint.
2. Remove and re-add fingerprints: Settings > Touch ID & Passcode > Enter the passcode of your device > Scroll down to find fingerprints. Tap on the fingerprint, which you want to delete and then Tap on Delete.
3. During the registration process, make sure your finger surface gets scanned and try the different angle.
4. If your finger is sweat or there is liquid on your iPhone, you need to wipe both your finger and your home button.
5. resar or iPad: Hold down the Home and Wake/Sleep buttons at the same time for about 15-20 seconds until the Apple logo appears.
6. If a restart does not help try a Restore connect your iDevice to computer and launch iTunes to restore your iPhone.