The fingerprint unlock won't work properly if you have your fingers sweaty, greasy, or have scars and cuts, added Apple, based on numerous testing scenarios it did with the Touch ID scanner, so just use a dry digit with uninterrupted fingerprint...Source: http://www.phonearena.com/news/Apple-calms-Touch-ID-privacy-concerns-again-says-chicken-wings-and-fingerprinting-dont-mix_id47382
