If you have created the Ishakawa diagram to identify potential failure modes and are trying to narrow the field to the root cause, there are several approaches available.
If you have created an Is/Is-Not (Kepner-Tregoe) Analysis, test the potential failure modes against each line of the Is/Is-Not. Your root cause will 100% match each line (unless you have multiple root causes).
As Claes recommended, you can also test them against the 5 Why. My experience is that you try the 5 Why first. Many of the simpler problems can be solved by this approach. If the 5 Why doesn't work, you try other approaches such as the Ishakawa.
If you do not have enough information to perform the Is/Is-Not Analysis, Enter the potential root causes from the Ishakawa diagram into an
FMEA. Rating each potential root cause will prioritize them for you. You can then address the highest risk items.
Note: I recommend the Is/Is-Not on all problem solving as part of the Problem Description step. It forces you to clarify the problem with all available information.