There are a few reasons. By fail, I assume you mean poor or no growth.
Poor graft handling, grafts can be squashed and die.
Grafts dying of dehydration. Out of the body too long without hydration.
Infection caught.
Necrosis scarring.
Not following post op instructions. Doing these too soon after your hair transplant. Out in the Sun and getting burnt, swimming, strenuous exercise as a few examples.
There are a few patients who for an unknown reason the transplanted grafts fail. It is referred to as the X factor. Like all types of transplants, the body can reject them.