iPhone Fingerprint ID Stops Working After 2 or 3 days

My girlfriend recently got an iPhone 6 and an iPad Air 2, both upgraded to latest iOS. The fingerprint ID stops working on BOTH devices after two or three days, at which point she can only unlock her devices by using her pass code. I have watched her follow the instructions to train her fingerprint scanner, and she is not doing anything wrong. She doesn't have any lotion or dirt on her fingers. The fingerprint functionality works perfectly on both devices for about the first day, and then by Day 2 it starts working less and less, and then after two or three days the fingerprint scanner does not recognize her fingerprint at all. She even tried just adding ONE finger, to make sure that the devices weren't confusing multiple fingerprints, but the same thing happens.

To see whether it could be the hardware at fault, we trained her devices to recognize MY fingerprints. I have had no problem whatsoever, even after several days of use. We then deleted my fingerprints, and trained for hers again, and the exact same thing happens -- the devices stop recognizing her fingerprints after two or three days.

So Apple, maybe you could look at the software? Perhaps there is something about certain fingerprint attributes which cause the scanner algorithms to corrupt the data.