I've had this problem on both my s7 Edge and Tab s3 ever since I've had them so either I have a problem with my fingerprints or Samsung's technology is weak. I'd like to test my print on an Apple device as they appear to be very reliable but it's a lot of effort and I'm opposed to Apple on principle.. I guess my FP works less than 10% of the time which is hopeless. I've done all the usual stuff recording them mutliple times etc at leasrt three times over.
Anyway as others have said Samsung Pay is a non starter if the FP reader is unreliable.
Its a bit worrying as apps are increasingly dependent on Fps.
I'm still having problems but I wonder whether it is because I suffer from dry skin on the hands. Two activities make the FP sensor perform less reliably, and often not at all. One is gardening, and the other is having a long soak in the bath!
Separately a friend of a friend who has recently acquired a second-hand S7 has apparently heard that the FP sensor works less reliably if power optimisation is not switched off. I cannot confirm this however because I already have powe optimisation switched to off.
That said the FP sensor works best with a very light touch, and I fear that invariably I press a bit too hard, even if not hard enought to depress the button (which of course one shouldn't do in this context).