Sometimes you have to ask Why five times to get to the truth.
What you hear at first is not always the truth. Or not the whole truth, anyway.
At first, people put the best story up front. They hope it will be enough and you won't go deeper.
Ask Why. Go deeper. Get down to the truth and get all the details.
If you want to prevent the problem from happening again, you have to understand what happened.
Be relentless. Don't accept what you hear at first. Dig deeper. Ask Why five times to uncover what really happened.
Without the truth, you will not be able to make proper decisions. You asking Why also helps staff think deeper when a problem arises.
If you want to learn more, read this Entrepreneur article about asking Why.