My model works until someone asks 'what if we hire two more people?' and I have to rebuild half of it by hand. I can get the numbers to add up, but wiring it so one change flows through cleanly still ties me in knots. Every tweak feels like defusing something.
More people experience this than they realize.
You can build a model, but making it flexible so one change flows through cleanly is hard.
If this sounds familiar, the Library can help you find the bigger picture.