Synchonization Problem in my displaying thread and computing thread

Hi, there is a problem when I try to run the following
codes. What I am trying to do is that when I run the
program in a "single-step" mode, it should execute the
addSolution function once each time, and then it should
wait until the user press the "Step" button.

The following code is from a thread doing the main
computation job. It is put into a loop, so that I am
sure if I run it without "Step" mode, it will be
executed at least 20 times based on my test file.

***************************************************************
stepExec = new StepSync();
...

if (numused == blockNum) {

      stepExec.step_run(); //I use this line to check whether it
                        // should pause or continue
     
      NewPuzzleFrame.solutionBar.addSolution(board_block);
      solutionNum++;
      System.out.println("solution Num = "+solutionNum);
}

***************************************************************
Here is the StepSync class. I use its functions to notify the
computation to continue or wait. It is somewhat like a
synchonization scheme.



***************************************************************
class StepSync {
public static boolean stepSignal = false;

public synchronized void step_run() {
  try {
    while (stepSignal == false) wait(); //pause
    stepSignal = false; //begin to run, reset the signal

    }catch (InterruptedException e) {}
}

public synchronized void step_cont() {
  try {
    if (stepSignal == false)
    {
      stepSignal = true;
      notifyAll();
    }
  } catch (Exception e) {}
}

} // end of StepSync Class

******************************************************************

In a seperate displaying thread, I have the following code to trap
a press on the "Step" button;

stepCont = new StepSync();
  JButton button3 = new JButton("Step");
    button3.addActionListener(new ActionListener()
    {
      public void actionPerformed (ActionEvent e)
      {
        stepCont.step_cont();
      }
    });
   
   
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OK. THE PROBLEM IS:
I debug this, and find that after two or three times of correct wait-notified
transitions, the computation thread will enter a state that it can not
be waked up by the button pressing. So the program has no response since then.

Why????