Apple Watch Fall detection

I have a Series 6 and am constantly given messages asking if I have fallen when bumping my arm, which is fine. However, I fell off of a roof last month (nothing broken!) and it did not detect the fall. My mother has a Series 8 and fell twice yesterday and it did not detect the either of the falls. We will both reset and check that it is on, but this feature is useless if it does not work all of the time.