Business opportunities lie in businesses that will solve future problems, and also those that solve current problems. I have a tendency to target the future problems, but learnt from a fellow entrepreneur to look at current problems too.
Short term cash flow definitely comes from solving current problems.The problem with current problems is that there is a lot of competition, which means you need to differentiate your business somehow, make it unique in the marketplace, bearing in mind that price is seldom a differentiating factor. Instead product and service differentiation are required. Or a unique approach to sell the product. This is what makes it difficult. Finding a unique way to differentiate your business.
So for short term cash do current problems, and for medium and long term cash do the future problems.