Finding the root cause of a problem can be tricky. There can be many potential causes and discovering the correct root can be a bit hit and miss. A common and effective tool to use is the 5-WHYs, where potential causes are each interrogated up to 5 times in sequence until you reach a practical limit of what can be influenced. If you need to visually group potential causes then a 'fishbone' or Ishikawa diagram can be useful.
There's also another technique which I came across many years ago, whilst attending a problem solving and decision making course run by Kepner Tregoe. The course contained a 'Swiss Army Knife" collection of tools for identifying and solving problems but the one that I have found particularly useful is a technique that can be applied to both specifying the problem and for testing potential causes.
It's centred around the concept of testing both what IS and what IS NOT. So rather than just asking what is occurring, you also ask what is not occurring but could be reasonably expected. This simple addition can help to clarify both the problem statement and potential causes. The technique is applied in four areas, being:
WHAT .…… defining the object that is deviating and the type of deviation.
WHERE .… defining the location of the object and that of the deviation on the object.
WHEN .….. defining when the deviation was observed over time.
EXTENT … defining how many objects and deviations.
The above information is first discovered for the problem definition. Then potential causes are generated from this information, by using 5-WHYs or something similar. Then these causes are tested against the detailed problem definition to find the best match. It's especially useful where there could be multiple root causes and a logical process of elimination is needed.
A search on-line will return many good explanations of the technique and examples of how it can be used. Many of these are concerned with manufacturing but the technique can equally be used in other areas to good effect.
As an example, I have noticed it being used in air crash investigations where there can be multiple possible causes. They do this by intuitively using the IS/IS NOT technique to define the circumstances surrounding the crash and then to systematically eliminate causes. Not one for nervous flyers to watch but fascinating nevertheless!