In general, my advice would be to be focused on the exact question asked. Typical cases will have 3-4 questions and whilst it's nice to draw links back to the original question (e.g. declining profitability), remember that you're being marked on being able to answer exactly what you've been asked.
On a question of declining profitability, I would keep it much simpler than going through "the 3 Cs". Declining profitability is an issue with revenue (price, volume, mix) and/ or cost (typically fixed/ variable) and I'd go from there. You've covered by definition everything to do with profitability when you've looked at revenue and cost, so once you know where the issue lies you can "peel the onion" and get closer to the real issue.