I can't see Swing JSeparator

I have this code:

  JPanel jpMainExample = new JPanel(new java.awt.FlowLayout(java.awt.FlowLayout.LEFT));
  jpMainExample.add(new JLabel("JLabel"));
  jpMainExample.add(new JTextField("JTextField"));
  jpMainExample.add(new JSeparator(JSeparator.VERTICAL));
  jpMainExample.add(new JRadioButton("JRadioButton"));
  jpMainExample.add(new JSeparator(SwingConstants.VERTICAL));
  jpMainExample.add(new JComboBox<>(new String[] {"JComboBox"}));
  jpOUT.add(jpMainExample);

But, I can't see the separator.

What is wrong?