Well with a mouse, you push the mouse button to draw. At this point the target falls to the ground paralysed for some unknown reason, so when you loose the arrow a second later, it misses. Its extra special when you then get a killcam showing the arrow missing your target, who is now getting back on his feet and running at you, while you stand there helpless until the cutscene finishes.
Temporary solution (until it can be fixed): Don't be too quick to try for the kill shot. You know there's a high chance that the paralysis, or other effect, will be triggered when the cursor is placed over the enemy, so wait.
Obviously this is a problem, but it's a temporary solution. I'm just saying I've not noticed it, or any of the problems archers have had with 1.5, even though I play an archer; but then I don't use enchanted bows. In fact, I feel that my archery has has improved. Waiting to take the shot might also reduce the misses in the kill cams