I've noticed that Touch ID is super brittle (at least on my own iPhone 5S), too. If your finger's slightly wet, that may be enough. Indeed, trying it now, if I

I've noticed that Touch ID is super brittle (at least on my own iPhone 5S), too. If your finger's slightly wet, that may be enough. Indeed, trying it now, if I lick my finger and then use it to try and unlock my iPhone a few times, it disables Touch ID and requires a passcode!