Treat the illness, not the symptom
When I attended the University of Chicago Graduate School of Business, we were taught to look at the underlying theory and system. The case method of teaching, whereby specific circumstances in which companies find themselves are diagnosed, was not the starting point. Instead, we started by learning economic, marketing and other theories and to understand the complex interrelationships that drive markets.
With this grounding, the approach when a business situation presents itself is to look for and address what underlies what is apparent. In other words, understand the illness, not the symptom.
For example, if an assessment of strategic gaps shows that marketing efforts are not producing sales, one's tendency might be to hire a new marketing firm or invest more in marketing or switch marketing channels - which would be addressing the symptom. With a theory and systems approach, one would start with the assumption that poor marketing results may be less related to the effectiveness of the marketing and more attributable to something more fundamental such as product characteristics or pricing or customer service or a changing demand profile or any other of the whole host of factors that can affect sales. The answer may still end up to be "lousy marketing," but with the theory and systems approach there is more confidence that that indeed is the answer.
One reason why strategic planning can fail is that the understanding of the situation in which the organization now finds itself and the gaps between that and its vision of an ideal future is too simplistic and looks only at "current symptoms" rather than what underlies these symptoms. It's easy to be simplistic about assessing what's not right and how to fix it, but success is more likely to come from a more systematic approach that does not take the presenting evidence as the answer in and of itself.