We capture relationships between variables using arrows.
The only connection type is some version of "causes". Thus the name Causal Loop Diagrams. Variations on "causes" are "affects" and "influences."
Make hidden or fuzzy variables explicit. Ask "how" one variable causes a change in another. That reveals missing variables.
Take some time to explore how this variable affects other variables. Sometimes the needed variable is already in the diagram. Sometimes you need to add them. Often times connections are more explicit when expanded to a series of causal variables.
The question: "Precisely how does this variable cause a change in the other variable" will find the missing variables.