When scanning your fingerprint, make sure that both the fingerprint lens and your finger are clean, and to use the center of your finger, not just the tip, don't move your finger and press it down hard enough so that your fingerprint covers as much of the sensor as possible.
If you keep having problems, first try rebooting your ZERO by holding the on/off button for 10 seconds.
If this does not work, you can wipe the device and import your backup. After 5 failed fingerprint attempts to log in, you will get the option to reset your ZERO. When the device reboots, define a new PIN, new fingerprint(s), import your backup key and add your coins to the dashboard again.
If you want, you can disable fingerprints. See "Can I use PIN-code without fingerprint?".
You can always contact support@ngrave.io for assistance.