Not knowing how old the string is, that would be my first guess, string stretch or a broken strand under wrap. Nothing wrong with your setup, of course there are faster and smoother shooting bows now on the market. I have seen this before when the string stretches the bow gets out of tune. The arrow doesn't fly straight to the target, rather at some type of angle. When it hits the target, it does so at an angle and loses a lot of momentum, (what drives the arrow), and impeds penatration.