You might think that each person has a single personality, one image of who they project to others. The reality is that we all have many roles, and each role requires us to play a different persona.
A CEO can be decisive and assertive, but as a mother she is protective, and as a friend spontaneous and fun-loving.
The same is true in your team. Telling someone they're the new CFO doesn't tell them enough to allow them to do their job well. "CFO" communicates job title, but it doesn't say anything about the role.
Do you want them to act as venture capitalists, supporting risky but potentially lucrative internal projects? Or does the current strategy call for tighter control on spending? Maybe you suspect fraud, and need your CFO to play the role of detective. Each of these roles requires the CFO to project a different personality.
Next time someone on your team isn't doing what you expect, have a conversation with them on the role you want them to play. And watch them transform in front of your eyes.