Problems with multiple causes are often "perfect problems." All efforts at solving a perfect problem fail because there is an underlying cause that is hidden.
One way to find the cause of a perfect problem is a multiple cause diagram. Write the problem at the bottom of a page of paper. Then start adding causes. Keep going until all the causes are revealed.
One of them will be very difficult. You won't want to face it. That's the one you have to fix.
Here's an example from the Open University of a multiple cause diagram.