Ask yourself why recursively about a problem 5 times. This will lead you to root cause of the problem. It will take you down the cause and effect relationship till you reach a deep enough point.
Problem: A project wasn't launched on time.
- Why wasn't the project launched on time? Because there was a missing piece.
- Why was there a missing piece? Because the team didn't know how to implement it.
- Why didn't the team know how to implement it? Because they didn't know how to.
- Why didn't they know how to? Because they were not properly trained.
- Why weren't they properly trained? Because no training is offered for this particular piece.