Why can't I tune a real control system even though I know the theory?

Theory is a clean world, no noise, no worn bearings, no sensor that lies a little. The real system has all of that, and the gap between the equation and the machine is exactly where tuning lives. Knowing the theory and struggling to tune is not a contradiction. It is the normal distance between the book and the bench. Tuning is a feel you build by hand, not a value you calculate once. It comes from turning a knob, watching the response, and turning it back, hundreds of times, until the behavior of real hardware lives in your fingers. The theory tells you which direction to turn. The repetition tells you how far. Pick one loop and tune it slowly and deliberately, changing one thing at a time and writing down what each change did. Do not chase the perfect number. Chase understanding of how this real system answers you. Do that on enough systems and tuning stops being a mystery and becomes a sense.