If the bear were not found, I would say they were bad hits, and not proper shot placement. Bear are not hard to kill what so ever. If you hit the heart or lungs, the bear will die quickly. I have seen people using everything from shotguns with lightfield slugs to 338 Federal with 200 gr. Hornady soft point bullets to a 45-70 with a 510 gr, hard cast bullet loose a bear. The reason was poor shot placement. After talking to the people, and them explaining the position of the bear and where they were aiming, it was oblivious they made a bad shot.
People always want to blame the gun or the bullet, when it is there shot placement that was the problem. I have seen where most people shoot to far back on black bear, or shoot through only one leg and never hit the vital area.
I always aim for the front leg or right up close to the front leg, I want to break down at least one leg and get into the heart or lungs. Every bear I have taken has not made it more than 25 yards from the point of impact of the shot. I have taken 3 bear with handguns, and 3 with a rifle, and my bullet of choice is a good hard cast bullet. I like 2 holes.
The Hornady LEVERevolution should of worked on his bear.