If you are hitting high and shooting for just behind the shoulder/leg you could be catching the edge of the shoulder blade. Only shooting 50-55 pounds, catching that bone will slow the arrow down. Also if you are hitting a major bone (shoulder ball and socket joint) on the opposite side ur arrow will not pass through.

I shoot 65 lbs with maxima 350's with no trouble. I have taken two shots that went through most of the length of the deer and the arrows were in the dirt on the other side. I would recommend uping the poundage of the bow. If you do that you will most likely have to switch to a heavier arrow. The increased speed and the heavier arrow will both aid in penatration.