The biggest thing I dislike about archery is the distance you have to be from your enemy before your arrow will score a “hit”. You can have dead aim, and still “miss” your target. (It doesn’t actually miss, it just doesn’t “hit” either, presumably because of the distance.) I tried an experiment the other night, went just outside Whiterun to one of the Mammoth/Giant camps around there. I took position on some rocks looking down towards the camp, and started shooting at a Mammoth. I was clearly in what I thought should be “range” of the Mammoth. It was easy to aim at as they are so dang big, so accuracy was not an issue. However, I took shot after shot and not one landed home. I kept inching closer until I started landing shots. I had to get way too close in my opinion to hit the dang thing. And when it finally hit, it alerted the two Giants and another Mammoth to my whereabouts, and I was subsequently pwned a mile above The Throat of the World, lol. Even though it is my preferred style of play, it still makes playing a stealthy archer a pain, because shots that should easily hit your enemy simply don’t register as hits. And when they do finally hit I seem to have to be so close that it alerts everyone to your presence, when it should not have.
Simple explanation for this one. There
is a maximum range, and it isn't particuarly far. Your shots probably are accurate and may even appear to hit the mammoth, but for whatever reason Bethesda made arrows stop causing damage beyond a certain range.
If you're on PC there are some .ini changes you can make to increase the max range. Can't remember what they are but using 'search' on the forums for 'archery' should yield some answers. In fact, I think it's posted earlier in this thread.