Fingerprint is not accepted when you first boot your device. similarly, if you use a third party application to lock your phone, it'll ask for your pin or password instead of fingerprint.
We are sorry to hear that is happening but we want to find an answer for you. Please let us know some more about what you are experiencing. What exactly happens to your device and are you doing anything specific when it happens? The more information that you can provide, the easier it will be for us to come up with a solution. We may serve you better/speedily, if you can provide logs by registering Error Section available in Samsung community.
