BorderLayout.CENTER doesn't appear

Question

Why doesn't uPanel3 appear in the center of the JFrame (aFrame) even though I am using BorderLayout?

 uPanel3.setLayout(new FlowLayout());
 uPanel3.add(background);
 aFrame.add(uPanel3,center.CENTER);
 //FlowLayout for uPanel2
 uPanel2.setLayout(new FlowLayout(FlowLayout.CENTER));
 uPanel2.add(errorLabel);
 aFrame.add(uPanel2,south.SOUTH);