One of the chief and primary reasons that a person never responds to the call is because of their fear of uncertainty. Respond to the call? Yes, I firmly believe that everyone has hidden within themselves the potential to be something great, they have been called by life to achieve something in particular. If they were to pursue that then they would achieve greatness. However, it always requires the plunge into darkness, the step into a dimly lit passage. The unknown, where risk is uncertain, outcome is not predictable. This is where a person accepts the calling. It is fear of unpredictable outcomes that keeps people from doing so and as a consequence never achieve what their calling was in life.
So the answer is simple. Step into uncertainty. Take the plunge. Go for it. Do what you know that you need to and are unwilling to. Take the risk. This is the road least travelled. Good luck weary wanderer. I will see you on the other side!