In my 20+ years of manufacturing I have read and/or worked on hundreds of 8Ds. I would venture to say that less than 10% of those led to effective solutions. The biggest problem in those ineffective 8Ds was the problem definition. 

If the problem is not properly defined based on facts and data you may be leading your team down a path that will yield less than optimal results. 

Think about it… how many times have you read an 8D with a problem definition such as “None-Functioning Motor” instead of the more precise definition of “Cracked magnet inside motor”.

Starting with a poor problem definition can make it extremely difficult to find the root cause without assessing every component and every process tied to those components (extremely time consuming). However, with the smaller scope in the example of the "cracked magnet" the focus of the team would be around the magnet and the factors and processes that influence it.

The best 8D methodology I have had experience with was the Faurecia QRCI. Their methodology forced a re-definition of the problem after the initial evaluation of the facts and data regarding the parts. It went as far as placing the redefinition after D3 (Containment) in order to force the utilization of the containment data to further narrow the problem. In the QRCI methodology, the 5-Why process does not begin until the problem has been properly defined.

The logical question to ask is why is this so difficult to do? In my experience the biggest problem has been timely and un-obstructed access to the “Scene of the Crime”. In many cases the parts are removed from where the issue was detected, the parts are repaired, tampered with or even lost. This significantly limits the ability to gather the facts and data, leading most individuals to fill in the gaps with assumptions and more likely than not, defining the problem too vaguely, or incorrectly.

If you place a priority on evidence gathering and put procedures in place such as stopping the production line when a defect is detected, your ability to properly define and consequently find and effective solution go up significantly.

- More on Problem Definition:

The Key to Innovation: Problem Definition (Part I)