Nice tips from Josh Betz on how to make Touch ID more accurate and responsive.
Go to Touch ID & Passcode in the Settings app. It will ask you for your passcode to change Touch ID settings. From here, touch one of your added fingerprints to the Touch ID sensor. You’ll see the entry for that fingerprint is highlighted temporarily, which means it just recorded more data. Do this for a while, walking it to different parts of your fingerprint, and you should notice Touch ID start unlocking your phone faster than before.
I do that for the same finger, but with multiple entries to Touch ID settings (you can have up to five), and the only time it fails to unlock my iPhone is when I’ve been eating something and my fingers are sticky.