Highly skilled managers in technical and professional roles are used to understanding what they do and how to do it. When it comes to leadership however they often get stuck at the declarative stage. They know the theory and what they should be doing but do not move into procedural or actual doing mode. They almost think it is enough to just know it all!
However, any skill being learned - including leadership - starts out as declarative knowledge. For example, when learning to play tennis, you learn about the rules of the game, where to come into contact with the ball on the racket, how to position your body for a backhand stroke etc, etc. This is all very useful declarative knowledge
Putting this into practice helps you gain the skills to transform a series of declarative knowledge into procedural knowledge. The skills cannot be learned simply by being told or reading a book. You gain the skills only after actively putting them into practice and being monitored by a coach who provides feedback to allow reflection to take place.
So, for leadership, these managers need to realise that it is not enough to know all about what they should be doing. This is clearly very important in the learning process, however, they must translate this knowledge into practice by doing it!