Interviews are testing two skills related to this:
1) Prioritizing analysis. Ensure that you make it clear what you're looking into and why up front, but that you have a hypothesis in mind. Rather than "which should I look into", say "If our sales are going down, maybe that's because of changing customer trends, so I'll look into that first. What do you think?" Similarly, ensure you justify each bucket in your framework based on the clues given in the prompt and based on your clarifying question.
2) Working collaberatively and taking hints. In general, they're not going to give you the answer, but ensure you read their responses. If you're like "i want to look at customers!", and they're like "um we're not that interested in it", then take that as a clue. They're job is to ensure that you make the most efficient use of the 30 or 45 minutes you have on the case, and so it doesn't do them much good if you run off in the wrong direction for a while