It's hard to transition from individual contributor to manager role. Becoming a manager is equivalent to applying for a brand new job. As individual contributor, the person is measured by how much work he completed. When he becomes a manager, he is no longer measured by how much he got done. He will be measured by how much his direct reports were able to accomplish.
There is a world of difference in doing what you do very well vs. motivating someone else to do very well. It is entirely different kind of art. Understanding how to be successful as individual contributor helps. But that's just one of ingredients to become a successful manager. Manager has to inspire, motivate and coach his direct reports to get better at what they do.
