The best way to develop an effective workforce is to hire well. That is to say that you should hire well then pay well, train well, and promote from within. How else are you going to retain these fine folks? One of the challenges of promoting from within is that the skills that made someone successful at one level may not be the same skills required for success at the next level. This issue is also known as the Peter Principle which, as you may well know, states that people are promoted to their level of incompetence. Competence, however, is a variable condition. Think about it, if you had your current position 10 years ago, would you have been able to handle it and do it as well as you do now? Competence is not fixed and is based on the knowledge, skills, and ability at a given point in time. Change any one of those and the game changes.
Developmental psychologist Karen Adolph studies learning and cognition in infants and toddlers and I think her work is relevant here as well. She found that as babies learn to crawl, they learn about edges and how it would be dangerous to crawl off an edge. That is to say they know the rules of acceptable behavior at that level of development. However, as they transition to walking, that knowledge does not translate. Even though they knew the dangers as crawlers, as they upgraded to walkers, that previous knowledge was no longer used as they navigated the world.
As you promote from one level to the next, you can’t assume that because they thrived in their previous position that they will thrive in their new position. It is a different set of rules. They can learn them, they just may need to some time and some coaching so they can learn the new rules.