paintComponent() isn't called after calling repaint()

Recently, i was practicing to make a small plugin. the simplified logic is probably as follows:

@Override 
public void actionPerformed(AnActionEvent e) { 
Loading loading = new Loading(); 
loading.show();

//some other operations 
XXX; 

loading.hide();

Methods methods = new Methods(); 
methods.showMethodsDialog(methods, dataSource, project.getBasePath()); 
}

Loading.show() is simplified as follows:

public void show() { 
JFrame frame = new JFrame(); 
LoadingPanel loadingPanel = new LoadingPanel(); 

frame.setContentPane(content); 
frame.setSize(220, 240); 
loadingPanel.setSize(200, 200);

content.add(loadingPanel, new GridBagConstraints(0, 0, 1, 1, 1.0, 1.0, 
GridBagConstraints.CENTER, GridBagConstraints.BOTH, new Insets(0, 0, 0, 0), 0, 0));

frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE); 
frame.setLocationRelativeTo(null); 
frame.setVisible(true);

loadingPanel.start(); 
}

LoadingPanel is a custom ring progress bar from google, it's core method is enable a thread to invoke the repaint() method recursively, as shown below:

@Override 
public void run() {

//other logic 
XXX;

while (!Thread.interrupted()) { 

//other logic 
XXX;

repaint();

try { 
Thread.sleep(inRamp ? 10 : (int) (1000 / fps)); 
} catch (InterruptedException e) { 
break; 
} 
Thread.yield(); 
} 
}

The problem now is that the LoadingPanel in Loading dialog does not display until Methods dialog shows. When i debugged, i found that the override paintComponent() is not invoked until the Methods dialog displayed, although the repaint() method is invoked all the time. I feel this is strange. 
Thank you very much for the help you are about to offer.