Start with a chart of the history of a resource whose growth you want to understand and control.
Identify, separately, the history of its inflows and outflows. You may have to do some investigation and reasoning. If, for example, you know your historic staff numbers, the change from month to month is the net inflow. You may not know the loss rate. However, you may know the hiring rate; in which case the loss rate is the difference between this hiring history and the history of net staff changes.
The key variables you now want to explain are these separate inflows and outflows.
Discuss the most likely factors driving a flow and find or estimate how these have changed over the same history. How many new staff did you try to hire month by month, and how many did you succeed in hiring? (Target hiring is clearly likely to influence actual hiring!) How have you changed starting salaries, for example, and how have your competitors’ starting salaries moved?