How do I diagnose a hidden plumbing problem?

You can follow a diagram, which means you understand how the system is supposed to work. Diagnosing a hidden problem asks for something more: figuring out where reality has departed from the diagram, without being able to see it. That's a real leap, and being stumped by it now is just the sign you're standing at the edge of the next skill. Hidden problems get solved by reasoning, not X-ray vision. The water shows up in one place but starts in another. The good diagnostician works backward, asking what could produce these symptoms and then testing each possibility until only one remains. You don't need to see through the wall. You need to think through it, ruling out causes methodically until the answer is the only thing left. So when a problem hides, slow down and list every possible source, even the unlikely ones. Then test them one at a time, cheapest and easiest first. Each thing you rule out narrows the search. The problem behind the wall isn't magic. It's a puzzle with a limited number of answers, and patience finds it far more reliably than luck.