Do problems occur continuously or sporadically?
Do problems occur cyclically?
Without prejudice or preconception, consider as many potential cause factors as possible. A helpful tool for this may be the use of the brainstorming fishbone to help put your causes into categories.
Example: you can use the 4M: Man, Machine, Method, Material.....you can even add a fifth one Environment.
Confirm with facts through Genchi Gembutsu
You have a cause when you can show there is a link between the existence of your problem and the existence of another occurrence or condition.
The best way to demonstrate a Cause / Effect relationship is to remove or block the cause. If the problem goes away you have found a Cause of the Problem. This is how you check if you have found the root cause of the problem
Pursue and specify the root cause by repeatedly asking “Why?” and building a chain of Cause/Effect Relationships to the root cause
Stop when you believe you have reached the root cause and check to see if it this cause is effectively counter measured will it address all the causes up the chain?
When you are in the middle of this investigation listening to others, please beware of clarifying whether you are hearing an "opinion" or actual "facts". Example: It is cloudy outside, so it will rain today. The fact here is that it is cloudy outside, the opinion is that "it will rain today"