Answer: Your question ‘What exactly blocks one from becoming Great’ is easily put than answered. In the first place, the word ‘Great’ is understood by people in different ways. One may be great in wealth, in immense power in the world, in learning, in character, in spiritual attainments and in many such things.
It is perhaps the power you bring with you from your efforts in the past lives and the concentrated effort you put now in the direction you want that helps you attain the fulfillment of your ambition. I do not know whether there is any force like gravitational force that blocks one’s way to go up. In our bodily life there is one environment which may be either hostile or favourable. In our moral and spiritual life there are the tendencies we bring with us from our past lives, which are either favourable or otherwise. But there is the power of the Atman (your own intrinsic infinite nature) within all, with which we can battle against these blocks.