I have always found that using a team approach can help greatly with determining the root cause of a problem. By getting multiple viewpoints it is easier to see what the potential problems could be so that you can investigate what the actual root problem is.
It is also important to understand a good root cause analysis tool to use. For simple problems I have found that the 5 Why approach (asking why did this happen until you get to a base cause) is very useful. For more complex problems I like to brainstorm using a fishbone diagram (as can be seen in the linked article). Here you go through the different potential causes from each category that could lead to the problem, and you can then determine which are most likely and eliminate them until you find the root cause.