There are times when trying to determine the root cause of an issue that the root cause cannot be found. The issue or the environment is too complex to separate out the issue.
In these cases, one approach to take is to address any smaller issues related to the issue or the environment. Once these issues are addressed, the root cause will exist in a situation that is less complicated. The addressing of the issues will have made the situation simpler. Now the issue will be easier to identify or other related issues can be identified.
By identifying and addressing related issues you are giving the root cause less space in which to hide. It is like trying to find something in a mess. Clean up some of the mess and it will be easier to find. The more mess you clean up the more likely you can find what you are looking for.