Most product failures don't come from picking the wrong problem but from never reaching its root cause. Eyal Pinko, a VP at Taboola, estimates that 90–95% of failures stem from a weak definition of what causes the problem. His tool is the Five Whys, a method that originated at Toyota: you keep asking "why", roughly five times, until you move from the symptom to the root.
For it to work, Pinko says, you first need to understand and define the problem with data, run the process with a cross-functional group, ask open questions and rely on facts rather than guesses. You stop when you find a systemic problem that will recur, when you get the "eureka" feeling, or when you hit a dead end.
Shuki Aharonovich of Augury uses an Opportunity Tree: a living document that starts from a metric and breaks it into problems and opportunities down to atomic problems, and doubles as an alignment tool with stakeholders who jump to solutions. Roee Froman counts breaking a complex problem into small, digestible pieces among the most important skills a product person has.