Your question is Root Cause Analysis. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Describe a time you were given a situation and had to identify the root cause of the problem. Use a real example from your experience, explain how you separated symptoms from underlying causes, and describe how you validated your conclusion. Cover the stakeholders involved, actions taken, and the outcome. Be clear about what you personally owned and what you would do differently.