you first need to isolate what numerical driver causes your profit decline. Revenue is too high level, you need to find out whether it is an issue with pricing, or the product mix, or the quantities. If, e.g., lower quantity is the problem, then you drill deeper to understand the concrete numerical driver (e.g., the average number of items per purchase has not changed, but the number of purchases has gone down --> then you drill deeper to understand what is driving this --> e.g., the number of customers has not changed, but their average frequency of purchasing has gone down --> this is the numerical problem driver! You isolated it just by means of a driver tree).
Once you have isolated the problem driver (WHAT is the problem?), then you check on the qualitative reasons that might have caused this very problem driver to develop negatively (WHY does the problem exist?). You exclude all other areas of the tree because they are not relevant. This is how you run effective and efficient diagnostics. This second step of qualitative analysis might indeed require some extra structuring once you reach it!