In our opinion, fingerprint readers are one of the best features to be added to smartphones in recent years. Being able to keep your phone locked down without wasting time entering a PIN or password is pretty dang convenient. Unfortunately, that convenience can quickly turn to frustration when your fingerprint reader doesn't work like you expect.
If you want to make your fingerprint reader a little bit more reliable, an easy solution is to register the same fingerprint twice. Most phones let you store up to five fingers, so, if you've got an iPhone for example, you might want to register both of your thumbs twice. This can be especially useful on older or cheaper devices with lower quality fingerprint readers.
On an iPhone, you'll find your fingerprint options by opening up the Settings app, tapping "Touch ID & Passcode", and then entering your PIN. Here you'll be able to add extra fingerprints by tapping "Add a Fingerprint".
On a Samsung, you'll find them by opening up Settings, scrolling down to "Lock screen and security", and then selecting "Fingerprints".
These settings should be similar for most Android phones with a fingerprint reader.