I would argue that there are two main types of factors that can make a leader weak.
The first of these factors is ignorance. People who have studied leadership know that there are a variety of different styles of leadership that can be utilized. They also know that different styles are more or less effective in different types of situations. A leader who does not know about this research is at a disadvantage. Such a person’s leadership skills are likely to be weak because they will not use the right kind of leadership style at the right time.
The second of these factors has to do with the person characteristics of the leader, which can cover a lot of things, like inaction, complaining, poor communication, lack of ethics, and more. It takes a certain type of person to be a good leader. The person has to be willing to use different styles of leadership at different times. They cannot simply say “I’m going to do it my way all the time.” A person who is too stubborn or too inflexible is not likely to be a good leader. Neither is a person who does not have the innate ability to do things like projecting caring or authority at the appropriate times.
Leadership is typically weak, then, due to a lack of knowledge and/or a lack of the appropriate personal characteristics.