My Touch ID is forgetting my fingerprint

My I phone 5S Touch ID is forgetting my fingerprint and is doing the same thing on my mums I phone 5S too? Why is this is there a way of stopping this? We both only have one finger assigned to the phone each and the I phone is still not recognising the fingerprint on either phone, I have restarted my phone to try and start fresh but it makes no difference please help, we got the i phones new and not long ago either 😟 Hope you can help