Lots of issues here, IMHO. I would agree with earlier comments regarding where you believe you are hitting the hog, and a review of hog anatomy may be in order. Another issue telling exactly where you hit the hog in the heat of battle is a difficult proposition. You said that most shots achieved complete penetration of both lungs, but that can only be said with confidence for the animals that you recovered.
It would also be nice to know the size of the animal you are shooting, as well. Most people shooting hogs don't get a complete pass through, but having two holes makes a huge difference in the blood trail, IMHO. I have shot complete pass through with well placed shots on large Texas hogs already and had a scant blood trail, so there are no guarantees. A friend of mine had a quartering away shot that grazed the heart earlier this year, and the hog still went over 200 yards. Fortunately, there was snow on the ground and he was able to track it. One hole doesn't always give the best blood, especially on a shot where the entrance is below the diaphragm and can clog up with fat. I am not going to turn this into a heavy versus light debate, but having a little more energy in your setup would give you a better opportunity for two holes as long as you can shoot it accurately. Another thing to consider, if you are able, is a tracking dog. If you are sure that you are making lethal hits but are getting a poor blood trail, a tracking dog can be an invaluable tool.