Android Phone Question To Any Experts [View all]

Have an LG Android phone.
I have it locked & 2 fingerprints recorded.
But, seemingly at random, the phone will say I can't use my fingerprint and I have my PIN.
I can't find anything in Settings that has anything to do with requiring PIN over just fingerprint.
It seems small, but it seems to happen more when I'm trying to turn phone on to get a text or check something on line.
So, the inconvenience is pumped up a bit.
My questions are:
Is this normal on Android?
Is there a way to change it?