Richie

First of all, take a lot of pictures, some up close to the observed defects. Your Hardie Plank siding system needs to be totally replaced. It never should have been power nailed. When power nailing, an equal amount of force is applied to all nails. Hardie requires all nails to be driven into wood backing such as studs or plates (not just through sheathing). When such backing is missed by a nail, too much force is applied and the nail is over-driven, cracking and/or breaking the plank. I can guarantee that the problem exists in more areas than you have already observed. I am a materials, means and methods expert and consult on many construction claims issues.