5.56 performance can vary widely depending on bullet selection. It's complicated a bit by the various twist rates of the rifles available (i.e. you have to pick something to accomplish your goals that will also be adequately stabilized by your twist rate).
If the typical penetration standard is the goal (12"-18"), then choosing 55 gr TAP doesn't make any sense. Since it uses the VMax bullet, I wouldn't expect adequate penetration from it - and that's what they got, inadequate penetration.