5 Why, Current Reality Tree (CRT) or another method?

Firstly the 5 should not be seen as a literal 5 but rather an indication that you need to delve deeper into the answer you receive. The 5 why relies very much on common sense, intuition (call it what you want), the ability to know that you have touched on the root cause. Obviously it is called 5 whys because 5 seems to be an average depth to get to a sensible conclusion.

Whenever I find an answer that cannot be controlled I know that somewhere between the first and the current why we have missed something (or overshot the root cause) and I would go back to find it.

One thing I've seen suggested before is asking the same question 5 times. I cannot really see how that would be efficient in any way because basing the next why on the previous why is what moves you towards the cause.

But as I said in the beginning I would not advocate that the technique name is seen literally but rather as an indication that you need to question something down to a level where a change will make a difference.

CRT and Fishbone is something I will use only for more complex problems that require a visual model.

Dear Anton
Yesterday I read an article on a blog that uses the term 5 Whys applying it to communication:
What, Why, When, Where Who by joining How, introduced by Rudyard Kipling

I remembered this topic about 5 Whys, created by Sakichi Toyoda to go to the root cause of the problems.

https://en.wikipedia.org/wiki/Five_whys