Usually it's because marketing and sales are working from different plans. Marketing may be attracting people who aren't a good fit, or sales may not be following up on the leads it gets. Common causes are an unclear ideal customer, mixed messages between the two teams and no agreed point at which a lead passes to sales. Fixing the strategy behind the spend usually matters more than increasing it.
The ROE finds where your spend is leaking and builds one plan that turns it into revenue.